Any tire shop should be able to find you that tire size in winter tires. Are you planning on buying online? Check these sites, just plug in your vehicle or tire size, and some brand name tires will pop up. For instance, Toyo makes Observe G0-2s in your size, and those are a great tire.
www.tirerack.com
www.1010tires.com
It won't hurt to change the tire size, but just make sure the diameter of it is within 1% of your old tire. This is ensure that your speedo isn't thrown off, and that your ABS (if you have it) works correctly. Try this site for other sizes.
http://www.miata.net/garage/tirecalc.html