The best way is to find somewhere that has a Bod Pod - but few places have them and it'll cost you around $50 or so. But it is the most accurate.
Calipers - probably at most fitness supply stores. Probably a supplement store too. Calipers are unreliable for EXACT BF%, have a high percentage of error. And most people (trainers) suck at taking BF measurements. But can be useful for tracking progress. Oh, and the same person should always do the measurement or else the result is pretty much useless for tracking purposes.