Here's an example of a 35mm slide case that is currently listed on ebay for an opening bid of .99 cents.
http://www.ebay.com/itm/35mm-Metal-Slide-Storage-Case-Smith-Victor-/290626851121?pt=LH_DefaultDomain_0&hash=item43aab5d131

There are many other types and sizes too. Do an ebay search "35mm slide case".