Actually they used to make single trigger frames for the spyder, might be able to find one on ebay or something. I don't think they sold them sperately, but they had them on older markers. You could probably pick up a whole used old marker for cheaper then a new frame would be.
Edit: here we go http://cgi.ebay.com/CLASSIC-KINGMAN-...QQcmdZViewItem
Not positive the frame would work, but it looks like its around the same dimensions as newer frames. I don't think they've changed them.