There’s a guy on ebay who has them for $115 all the time. Just search on ATK 250 piston. Besides Terry, American DirtBike also has all the parts for the ATK 250LQ (Rotax 257). Rod kits, pistons, etc. Parts are still pretty plentiful for these.
BTW You can even buy Rotax 257 pistons on Amazon.com!!! I got 4 last year when they had a special for $85 each w/ free shipping. Icluded ring and wrist pin but not wrist pin bearing which is also available on Amazon. Not hard at all to find. Now the rotary version model 244, that’s a bit tougher……..
They have both std and two oversizes on ebay and Amazon. Up to 68.00. If not listed, ask seller, he has them I know. But all are Wiseco. If you want OEM, you have to go to Ives, Murphy or American Dirtbike.