If you want accuracy and reliability get a Dole 400B. They list for about $500. I have one that is 18 years old and is still a good tester. If you want a good hand held tester get a Dickey-john M-3G. It lists for $189. www.agri-tronix.com is one source.
Check out e-bay (I got mine there). There a bunch of them, includng a Dole 400 (current bid $26). Just do a search on moisture tester under ag equipment. Good luck!