I know a couple of nice slap solos:
The Reddings: The Awakening
The whole song is impressive. I personally think this is really hard. If you can't find tabs, try looking for them under the name of "Les Claypool," since he does this solo once in a while, too. This is a hard one - I'm still trying to get all the parts down.
Red Hot Chili Peppers: Coffee Shop
There are two bass solos in this one. They're both funky but one is slow and the other (later) is fast. Pretty hard.
Stanley Clarke: School Days
The slap bass line for this is awesome. It starts off with some stroking and then hits the main thing. And then there's this little interlude or something that uses slap/pop that sounds real bluesy. Not that hard, but if you try to mimick ol' Stan's fingerstyle solo in this, your fingers'll pop off and find a new owner.
Marcus Miller: Mr. Pastorius
Really nice, smooth, somewhat depressing song. It combines tapping and fingerstyle. Intermediate. It really comes together once you get the timing right and practice a bit.
Red Hot Chili Peppers: Aeroplane
This was my introduction to bass playing.

The intro is very nice and harmonic and the chorus is also nice, but I think this song really shines when Flea starts slapping for the verses. He also does a little fingerstyle funky solo. It isn't that hard, but I had a really hard time memorizing all the variations Flea put on the verse slap line.
That's all I can think of now. I'd bet you'll get a whole lot more from others.