Try Scriptlance or RentACoder. If you're looking for examples of implementations, greenmountainenergy.com has a good one, the EPA has calculator (and data tab

Try Scriptlance or RentACoder. If you're looking for examples of implementations, greenmountainenergy.com has a good one, the EPA has calculator (and data tables for download), sterlingplanet.com has one, nativeenergy.com has one, carbonfund.org has one, terrapass.com is another. Some of the data they're using comes from the transportation energy data book (or digital versions thereof), WRI GHG calculations or ISO 14000 series environmental management methods. The rest of the problem is the credibility of the verification of the demonstration of additionality of the offsets. Try to adhere to The Gold Standard. Hopefully you knew most of that already.