There are a number of cheap (ie. non-specialist) flatbed scanners which include a slide adapter. The low-end Plustek and HP models come to mind, both of which are under $300CA here. I'm sure there are lots more if you do a search. The quality of the scans is not 'professional' by any stretch, but still probably better than your local Kinko's.
Doesn't help if you already have a scanner of course - I don't know of any retrofit products, but I may be wrong as I haven't looked recently.