I can't find a slider phone and I really want one again. There is an abundance of people yearning for the return of the keyboard without losing screen size
Your completely out of luck on a slider as nobody is making them anymore. The only physical keyboard phones anymore are from BlackBerry and Koodo has no intention on carrying them at this time because they don't sell well. You can check out BlackBerry's webstore and purchase an unlocked one straight from them to use on Koodo just make sure it's a compatible model with Koodo's frequencies.
You can find a limited selection of sliders at Amazon.ca, even more at Amazon.com and eBay. Anything unlocked that is AT&T compatible should work fine with Koodo.