Just make sure the vermiculite you choose does NOT have fertilizers added to it.
If it says "organic" that does NOT mean it doesn't have fertilizers. Repeat.... it does NOT mean it is just vermiculite. It only means that any additives qualify as organic, which can include a lot of toxic things.
Buy a bag of the "plain". The brands like Miracle Grow will usually have the added stuff that you don't want.
I've always picked it up from my local ACE but I bought a huge bag and still have most of it left. There's SO much in one of those big bags.