NOS panels are about, but the problem is everyone seems to be squirelling them away and you can't blame them I guess. Your chances of getting hold of used inner wheel tubs in good condition are slim to say the least so I would agree with Steve and go with Expressed Steel.
If the edges are a bit crusty they'll be a nightmare to repair as the wheelarch lip is a compound curve. Far better in my opinion to put clean new metal in even if they need a bit of fettling.
As I understand, the inners are the same, but outers are different? Only need inners, so could be a result? Probably easier to repair off the car than on as well, thanks for the lead.