Here's a cheap solution to see you through the nicer summer weather: Beg, borrow or steal a 7-speed standard 26 inch mountain bike wheel - these are much easier to come by than a 650B. Mountain bikes get dumped at waste re-cycling centres daily. Because it's smaller than your 650Bs it will fit the frame, and you can use that for the turbo. You will need a slick tyre, but it doesn't need to be turbo-specific, any cheapo will do. You may need to unhitch the V-brake noodle so the pads don't foul the wheel. Obviously, don't use the brake on the turbo! You can then use your existing wheel/tyre combo to get outside for some real cycling.