Bill, you can also try asking on here for the specific line weights you want. I know you are in the sweet spot of weights. If you get lucky then you will not have to watch the classifieds like a day trader in order to get the ones you are looking for. Just do a "WTB 480 scandi and 510 skagit" in the classifieds.
I realize the explaination below is long, but it would have been exactly what I wanted to know back when I was in your situation, so I hunkered down and tried to lay out some of it I detail. It is long because the product situation w/regard to sink tips is in fact pretty complicated, as you are finding. You will seldom get a real breakdown of this information all in one place on here since it is kind of like the air we all breath.......
I will let you on a minor confusion that is good to know, though not a game changer: courtesy of the Rio
department of obfustication, and historical accident, there are at least 2 different ways of labeling sink tips. For tips with names like T11,T8 etc. the "T" stands for tungsten, and on that case the number stands for grains per foot, as in T11 weighs 11 grains per foot. A lot of the time people just learning about this stuff understandably get confused by the alternate (and more traditional) sink tip designation of "type". As in a type 3 sink tip sinks at 3 inches per second, or ips. There are a lot of people working in shops that dont understand the difference either. There is a "T" in both designations but they stand for different things, and the NUMBERS mean something different and so do not line up either. So for example T11 is roughly a type 8 sink tip. This information is on the little cards that often come with the sink tips, but a lot of the time it still confuses people. In practice most people have a complete set labeled one or another, and just shift "gears" until they get a satisfactory result and are fishing at the right depth, so like I said at the top it will not make a huge difference. But when you get to the inevitable situation where you have all kinds of mixed provenance tips in your bag the info helps to compair the apples and oranges. It it were simple you wound not have so many thread on here about "what tips do you use with this and that rod and line".
The other difference between sinktips that is good to know at the beginning, especially when you are buying them online is that the "T-stuff" has a fixed diameter, and as the number indicates the weight per inch goes up as the number goes up. Since the diameter of the tips remain the same this means the DENSITY of the tips goes up the larger the number. The sink rate depends on the density, not the weight, since the force down on a tip is proportional to the difference in the weight of the line and the weight of the water it displaces. It also depends on the friction (the force up on a sinking tip) of the water on the tip, which is complicated but "goes down the thinner the tip" covers %99 of it. There are other ways to make the density of the tips change, and the other obvious (and as it turns out useful) one is to keep the weight per unit lenght constant, and just make the diameter go down. This is the strategy use by the Rio
replacement tips. If you buy a set of 9wt 15' rio
replacement tips, for example, they will ALL weigh 129gr, but will get thinner and thinner as they go from the intermediate, to the type 3 to the type 6, and so on. So this means that between 4 wt and 11wt with floating, intermediate, and types 3,6 & 8 (color coded) there are actually on the order of 70 different tips when you include the 10' ones.
The advantage of the Rio
replacement tips is that when you have a set of them they will all be of the same lenght and weight, and that weight is more finely adjustable to fit your rod. The disadvantage is that they are more expensive. The advantage of the "T-stuff" is that it is cheaper, and you can even buy spools of it and make you own. Also, for the really fast sinking stuff like T14,17 and 20 it is the only option, as far as I know.
My PERSONAL opinion is the rio
replacement tips are the Cadillac of sink tips, or perhaps a better analogy would be like shoes that come in all sizes and widths so that you can get a custom fit, where as the T stuff is like flip flops where you have to be satisfied with S,M and L.
This would get way too long to cover everything, but there are also versaleaders, which can be awesome on light rods but are both cheaper, and can sometimes be more fragile. Then there are MOW tips which get their difference in depth by changing the lenght of the sinking part, but are more like the replacement tips in that they keep the total length and weight the same by incorporating a floating portion of varying lenght in front of the sinking part. I just realized as I have never had a complete set of one size of those that may not be exactly true as far as the weight is concerned.
IMHO, when you first start, using the Rio
replacement tips it may feel easier to learn to cast since all your tips in a set will all be of the same lenght and weight. You will still have to deal with the fact that denser tips will tend to sink faster WHILE you are casting, but all the other variables will be the same, and so easier to learn because you will not yet have internalized the required adjustments in your casting that will eventually become automatic when you switch to different lenghts and weights of sink tip. MOW tips have that advantage as well, but like flip flops the come in a more limited number of sizes. They ALL, including the T stuff, have their religious advocates.
Sorry for the lenght, but I post this here because this is the information I would myself have loved to have had, all at once and in one place, back when I was in your situation. I hope it helps.