There are certain kinds of slap sound that are very much compressor sounds - if you get a fairly versatile compressor (ie, one that has attack, release, knee and ration controls), you should be able to experiment with what works for you.
you're dead right that it's mainly about technique, and you might be better of spending your money on lessons to get some ideas rather than gear that may or may not work for the sound you're looking for.
the gear lists in Bass Player magazine can sometimes be instructive with regards to how to find particular sounds, but ultimately it's down to you finding your own sound (I know, that's not what you want to hear, but either I tell you know, or you find out the hard way... :o)
have fun
Steve
www.steve-lawson.co.uk