I know the stuff you're talking about, used to see it towards the back of Popular Home Mechx magazine, etc... I THINK it's called or made by a mfg called Beaver Industries. Do a search on the web for word combob like beaver, basement, floor, etc... one site, however, that actually talks about and shows pictures is: http://homecentral.com/HowTo/Und1T009.htm
Hope it gives you something to go on. BTW, my area menards and Home Depot know, some even carry it, as this same 'beaver' company also makes vinyl fences, probably same material the cove moldings are made of... Hope it helps... let us know how you come out.