They are fitted and require a prescription. I specifically asked my ortho for a CTI knee brace prescription; making sure he put the word "CTI" on the prescription. I think my insurance classified the knee brace as "durable medical equipment" so check with your your insurance and find out what their policy is on that type of equipment.
I personally spent 10% of the total cost on mine (after insurance) which was about 175 bucks.
While a knee brace doesnt guarantee that you wont demolish your knee or cause other injuries (since it strengthens the weakest link if something were to break) I looked at it like this: cost of knee brace: $175, cost of knee surgery $5,000 out of pocket. Yes I have done both. IMHO if I had a torn ACL I would be buying a brace...even if it cost the full $1,750.