I have a breeding pair of oscars that was a rescue from someone keeping them in a 55g tank. I got them around the 6" mark and for the longest time I really thought they were both females, since around the 9" marks I kept seeing eggs that were never fertilized. I got very lucky with my pair but that is not the normal.
The best way to find a pair is to buy 6-7 juvies and grow them out waiting for a bonded pair. The issue with that is it will require a huge tank to house these fish for a 9-12 months which means they will be around the 9-10" range. Then the second issue is having that many aggressive fish in one tank waiting for the pair. This will require alot of dedication with water changes every 3-4 days to keep the nitrates in check. The other issue is what to do with the fry, they can lay up to 1000 eggs per spawn, now thy all don't make it of course but it's not uncommon for 30-40 fish to survive. The last issue is trying to rehome large fish, I know in my area there are no LFS that will take any large fish like oscars. Sometimes you can get lucky and people will sell off a pair on websites like Aquabid. Good luck.