Here is a great resource for slides of different sizes and shapes. It is called Microscope Depot. Prices are very reasonable and they ship in just days. Here is a link to their sight:
http://www.microscope-depot.com/slides_cover.asp
An easy way to finish the edges of a slide ornament is to wrap the edges with Sticky Tape (double sided & very strong) then roll the taped edges in micro beads, glitter, or wrap in ribbon. I've tried all three and they turn out great.
Good luck!
Audrey