Most jewelry stores will be able to order a specific ring in your size. It may cost more, it may not, it depends on the store. If you've found the ring you want, ask the store manager if they can order it in you size. Otherwise, call the stores in your area and go to the one that does. Alternatively, get the closest size above your own and have it resized or use a sizing band (any jewelry store will have these)
How do you know your ring size is 2 1/2? You must have tried a ring on that size before (to know it fit correctly) -- so contact the person who owned the ring you tried on (or sizer) and ask them. Try google as well -- it sounds like you will have to get a ring thats custom made