I know that Apple has used Crucial (Micron), Samsung and Hynix RAM...I'm sure there are/have been other suppliers.
I've purchased both OWC and Crucial third-party RAM and never had a problem. But based on other people's experiences, both companies will take care of you if your RAM ever goes bad. Any of those three I cited in my previous post have excellent reputations amongst Mac users. That's not to say you can't get good RAM elsewhere, but those guys are definitely top-notch.