Here are the native turtles species of Ontario and you may not keep any of these turtles unless you get a special permit (which are hard to get):
Chelydra serpentina (Common Snapping Turtle)
Sternotherus odoratus (Common Musk Turtle)
Chrysemys picta marginata (Midland Painted Turtle)
Chrysemys picta belli (Western painted Turtle)
Graptemys geographica (Common Map Turtle)
Emydoidea blandingii (Blanding's Turtle)
Clemmys insculpta (Wood Turtle)
Clemmys guttata (Spotted Turtle)
Apalone spiniferus (Eastern Spiny Softshell)
You can keep everything else that is not native to Ontario. There are no native mud turtles in Ontario, and there are many different kinds of musk turtles that you are allowed to keep. Just not the common musk turtle (aka Stinkpot Turtle).
As for selling turtles. Turtles are usually imported from the US, and technically it is illegal to sell imported turtles and their offsprings. So, most businesses are not into selling turtles, usually private individuals. But some businesses will sell, but they keep quiet about it.
You'll have to look around for a private sale. I suggest you research the turtle you want before you start looking.
Cheers,
JJ