Scientific Anglers, Rio
and others make them. They are, essentially, tapered, clear intermediate sinking sink tips to which you attach your tippit material.
Not strictly true!
Polyleaders, from whatever company, are an intermediate form between a tapered mono leader and a fly line tip.
Polyleaders are usually untapered mono cored, with a tapered weighted coating and range from floating (truly a tapered mono leader only with no coating!), right through the range of sink-rates from slow-intermediate, intermediate, sinking, fast sinking and super or ultra-fast sinking.
Most polyleaders come looped at the fatter rear (fly-line) end, and have an exposed (uncoated) lenght of the mono core at the fly end, which you may then tie a loop in so that you can loop-to-loop attach you leader material to, or you can use a fine metal rig-ring to tie to the end of the mono of the polyleader, and then tie your leader material to the other side of the rig-ring. You can also, if you wish, do a usual connector knot between the exposed mono core end of the polyleader direct to your leader material.
The purpose of poly leaders, taking the UK 'by storm' (aka it's almost a "crime" to NOT use one!!
), is to more gently transition between the end of your fly line and your leader proper.
How ever DID we manage to fish at all before these companies decided to charge us £5+ per 5 yards (or less) of such materials??
There is also much chatter about 'better turnover' of flies, and the vociferous aficionados would have us believe in these bits of kit so much so, that anglers are almost encouraged to chop the fine ends off their otherwise good fly lines in order to squeeze one of the indispensible polyleaders on the end so that you can "fish properly".
I don't use polyleaders ... did you guess that????