DO NOT buy cheap ones, i.e. the ones at Harbor Freight or other crappy cheap clamps. Go with the Irwin Quick Clamps, Jorgensen ISD 3 or possibly the Bessey clamps. I have about a score of the Irwins, some of the Jorgees, and a herd of other Bessey clamps, but none of their new "one handed" clamps, so I have some familiarity with clamps. I have also had some of the clamps shaped objects sold by Harbor Freight. Note the past tense. Crappy clamps can ruin your day. Lowes, Home Depot, Menards, and your local hardware store all carry the good 'uns, and you can frequently get package deals on the Mini-Quick Grips.
:soapbox
Anti TSO (tool shaped object) rant off....