I tore my left ACL (righty also) in '05 and had surgery to repair it in early '06, and I'm on pace to bowl almost 2000 games this year... so I think I know more than most on this one :)
Seriously. You're really going to stress/injure your knee if you can't slide properly. Get a good pair that has interchangeable soles if at all possible. Also, what knee brace did you try? I use this one and find it helps a lot when paired with a proper slide on the approach.
That help? Fire back if there's anything I missed or if you have more questions!