The ring, bore and piston sizes must all match. A piston 0.010" smaller than the bore would slap quite noticeably, and its rocking would jeopardise the sealing ability of the rings. Normally the procedure is to measure the bore 1 cm down from the cylinder head mounting surface, and the piston diameter at the skirt, comparing both with the nominal dimensions of new parts. If the wear is substantial you have a choice between boring the engine you have, finding a second-hand one that doesn't smoke, or buying a new engine. Usually the second alternative is by far the cheapest and easiest. If your current engine's bore is smooth and not all that worn (which is usually the case), you can get a few more years of good performance by just replacing the piston rings.
So far as sourcing parts like rings is concerned, I think you'll find the Outdoorking online store has among the best local prices. You can compare that with overseas online suppliers. My experience so far has been that US sellers often (not always) have good prices for the items but truly awful shipping charges. In that situation I look at Chinese and Thai suppliers, whose shipping charges are usually around one tenth those of US suppliers. Of course you then have to check out the supplier, to ensure the merchandise is good quality. Essentially the Asian suppliers seem to mostly consist of very small businesses which list any item that some manufacturer is distress-selling, so there is no coherence to their product list, it consists of everything that is available. However there are also specialist suppliers who concentrate only on engine parts, with a sizable range available continuously. If I am buying engine parts, I only deal with the latter type of seller.
Obviously very few engine parts are made in Australia these days, so they are coming from the same place, whoever you buy them from.
