It may be cheaper to just get a smaller cog and keep your 40 ring, but if that doesn't give you a ratio you want:
To find compatible chainring- Check the bcd on your cranks. Sugino track cranks should be 144. Get 1/8" or 3/32", whatever you're currently running. Sugino, campy, shimano, salsa and many more companies make good rings. From my experience, you get what you pay for (up to a point) i.e. avoid el-cheap-o rings.
To find the right ratio- I'd test ratios on a geared bike and use what works best for you. Advise on the net is going to have quite a bit of variation based upon abilities, topography etc. That said, it's pretty flat here and I usually run around 44x16 up to 47x15.