My slap technique is really terrible but the first bass that made me sound moderately decent at it was a fretted Fenix (basically a Korean Squier) Jazz. While you're on Youtube, check out the reviews by 11coopbass: http://www.youtube.com/user/11coopbass/videos
He has fantastic ability (the most important thing) and tries out a lot of affordable stuff from Rondo and others. Mostly Jazzes, but a MM and Precision copy feature, amongst others. A lot of them are pawn shop finds for not much money. See what tone you like and start trying those basses out.
I can't emphasize enough that working on technique is worth more than choosing a particular brand or design. This video of his is one of my favourites: http://www.youtube.com/watch?v=Rb_UxNqhrFQ