Usually, activities like desert safaris are not pre-booked. Once you reach Jaisalmer, you should contact your camp/hotel or ask around locals. Alternatively, you could check out these camps in Sam where you could stay at!
I have done safari at Sam sand dune on 21st and 22nd November but I strongly recommend u dont go there... go to non-touristy region . At Sam sand dune, they r looting by making us full