Regarding essential oils, remember that you're going to get what you pay for. The best oils are not cheap (maybe $12-$100+ a bottle,) but a little goes a long way. And make sure you don't buy 'perfumed oil' or 'fragrance oil.' 'Essential' is the important word here. I dove into Aura Cacia's essential oils, as they're really affordable. But the scent wouldn't diffuse in a room and it fades very fast. I'm thinking you could always add these oils at the last minute just before you add the salts to the bath, and be ok. If they sit in a jar, the scent will probably fade. Hit up Amazon.com for Aura Cacia. If you want to go high dollar,
Organic Essential Oils & Organic Massage Oil from Young Living. If you enjoy the likes of that lavender bath salt you mentioned (I've used it too,) you'll LOVE the 'Peace and Calming' blend. Promise you that one.
I'm interested to hear your results.