For a moderately priced unit check out Parts Express, Radio Shack or Fry's. Search either spl meter or sound meter.... It is tedious to do it this way as you have to transpose your readings off of an A or C weighting. If you can spend a little more money there are computer programs, assuming you have a notebook, with calibrated mic's that will save you a lot of time and hassle. Check out the Dayton Audio Omnimic V2 at Parts Express.
I heard that there are app's that you can put on a smart phone but I'd be concerned with accuracy.
I heard that there are app's that you can put on a smart phone but I'd be concerned with accuracy.