How large a canvas do you want?
You can scan slide film on any modern flatbed scanner such as a Canon 8800F/9000F, and they will normally come with film strip holders and mounted slide holders. They're about £150 to £400 depending on what you want. The newer Epsons are more expensive but offer little in the way of resolution improvements for double the money.
You could look at a Plustek Opticfilm 8200i or similar which is realistically the most reasonable value and best resolution for £200-400, you still wont get much more than 2800 DPI and a slight dmax improvement over the flatbeds. It only scans 35mm film though, and has no platen so it can only scan film, not sure if thats an issue.
The quality however..I scan with an 8800F and its fine for web, 6x4s and occasionally 10x8. I wouldn't print any larger as the true optical resolution of most of these scanners is around 1600 to 2000 DPI. However the film base on slide films is denser and the dmax of slide films is enormous, way larger than most flatbeds will be able to cover so the quality will depend on how large you want to print and how much detail you want to retain.
If it's a few slides for a one off then send them to be scanned in a proper lab that uses something like a Flextight (3 CCDs, best quality outside of a drum scan) or a newer Noritsu scanner. Avoid places like Snappy Snaps as they usually scan with a desktop scanner.
