If you don't have ready access to a good used bike shop, or better yet, a bicycle co-op where you can root through a box of parts to take what you want, it's often cheaper and easier to simply replace the whole crankset in one go.
If you happen upon a middle chainring, don't worry about it being biopace. Round and Biopace chainrings can play together just fine. I have a Biopace inner ring with two outer round ones, and no trouble at all. As long as the BCD matches, and the tooth count is close you are good to go.