another “industry” acronyms is R2D - Ready to dye - more commonly used in Fabric world
Most yarns are not completely PFD because most dyes require the use of some type of mordant both to influence the color AND to stabilize it
We leave the rest of that conversation for others more knowledgeable than I to expand in other topics
These are my two preferred sources for PFD - not the cheapest, but in terms of Price/Value/Quality - well worth your consideration
(and they do not compensate me to say that, just an always satisfied customer for literally decades)
Both are a US company - with really nice people and excellent products
Kraemer Yarns - The Naturals collection
https://patternsbykraemer.com/collections/yarn/products/naturals/
http://www.brownsheep.com/about-us/
If you are interested in felting - they have some of the best products in a wide range of weights
http://www.brownsheep.com
Both have wonderful roving that I REALLY like for both wet and dry felting
and thier wool yarns all full beautifully.