Making your own bath oil is very satisfying and very easy. The most basic recipe is to mix your chosen base oil - Sweet Almond for example, with a small amount of essential oil.
Consider infusing an oil with herbs. Simply take your base oil - again Sweet Almond, Apricot Kernel or whatever you choose and infuse dried herbs or flowers in it. After about 6 weeks you will have an easy to make, totally handmade bath oil.