I get 490565 too, for standard bore. I got it from here:
http://www.stevemyersserviceinc.com/pages/OemParts?aribrand=PRO#%2fBriggs_%26_Stratton%2f100708-0128-02%2fCylinder%2cSump%2cPiston%2cOil_Fill%2f100708012802%2f91610002B

They seem to be priced reasonably here:
http://www.ebay.com.au/itm/BRIGGS-STRATTON-NOS-PARTS-490565-PISTON-RING-SET-STD-/280758359984

If you are having fun fixing an engine, paying $20 including freight for rings seems worthwhile to me.