I don't know if you found a suitable answer, but why not get a cheap synth, or maybe one of those cheap-o keyboards from Radio Smack? That way you have one piece of equipment to lug around instead of 2 or more.
If you want something inexpensive but with real synth capabilities, you can get something like a used Alesis Ion, or any of a large selection of good but not-too-pricey synths out there.
I'm not crazy about E-Bay, but people sell a lot of this stuff there, so that might be one place to check out.