Bob is a very good local rod builder here in Medford, OR; WillieGunn (Dr. Malcom Newbold) is in Banff, Scotland. One, or the other, should be able to tell you where to buy cork rings in either the US, or the UK. Both are board members so you can also just send them a 'pm.'
One other place to hit is "Angler's Workshop" in Woodland, Washington on Google. 95% certain they'd have a web site and they sell all mannor of rod building supplies. Been in business for at least 25-30 years so safe place to 'shop.'
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