The standard choice for an off-the-shelf two-speed transmission is the AndyMark Shifter in one of its flavors. They run from $330-360, depending on the model, and quite possibly saved our butts last year on the field. Their website is
www.andymark.biz.
If you're going to spend that much on transmissions, do yourself a favor and make sure you have the rest of your drivetrain set to take advantage of it. After all, a low gear isn't going to help you if you can slip your wheels with ease in high. My advice is to look at JVN's spreadsheet
here and fiddle with your numbers first.