Okay... it' sprobably a lot easier to just buy any of the effects listed that include slides, but since it can be fun to be cheap and experiment with found magic, here's something to think about:
If you happen to have an old vacuum cleaner you no longer use or have access to old vacuum cleaner attachments (maybe you run a vacuum cleaner attachment museum), look for the long, flat, hose attachment used to reach under furniture and into narrow spaces (it's called a "crevice tool":
http://www.totalvac.com/accessory/parts ... 0-3ES.html
If you can get one form a handheld vacuum (like the Shark), it'll be smaller, but a standard size one will work and give you a wider range of possible coins.
Cut off the ends to get the length you need; sand the rough spots; make sure there is no "lip" inside the catch the coin. There you go. Coin slide.
Of course, you could just buy some sheet metal and bend yourself a slide... but where's the fun in that?
Another alternative is to go to a dollar store that sells cheap magic. There's an effect in which a coin placed in a sliding drawer vanishes and reappears when the drawer is open and shut:
http://magic.about.com/od/libraryofsimp ... nslide.htm
You can find them for a buck or two. The outer case that the flat drawer slides in and out of is the right size for a slide. Just rip out the guts.