Check out A1 Photo Booth's "Unofficial Sparkbooth Layout Library". Joe has created this site as central place where Sparkbooth users can browse, download, share the layouts they have created. There are layout backgrounds, overlays, and SBL files created with the Sparkbooth Layout Creator.
Please leave Joe some feedback or thank him if you find it useful.
**UPDATE**: Joe has added How To Instructions and a place to upload your layouts