For small batches of salts you can buy oils at Michael's, HL & JoAnns. Get them in the section for making beauty supplies. They are not going to be the most high quality but I have gotten some lovely results using them.
My second favorite place to buy essential oils is the health food store. They are very pure and must be used sparingly.
If you want to be hard core and really getting into making beauty products you can make your own essential oils. This is a time consuming process and takes a lot of patience. It's also can be very Seasonal. I have a friend who teases me that she cans and I make oil during the Summer months.
If you have a perfumery near you they will sell you blends too.
I really don't mail order since I like to smell the oils when buying. I am finicky that way.
HTH!