You could try this page http://www.leeds.ac.uk/music/Info/RRTuneBk/search/searchform.html in Richard Robertson's Tune Book and do a search for all Shetland tunes. There might be some for you to add to you collection there.

Also, the ABC Homepage http://www.gre.ac.uk/~c.walshaw/abc/ may have links to specific collections of Shetland tunes. I can't get in to find out at the moment.

Jon