George,
Good questions! Your Hobby store should carry or be able to order Epoxy Finishing Resin. Brands such as Hobby Poxy Finishing Resin and Pacer brand Z-Poxy Finishing Resin are two that they can aquire.
However, it sounds like you found the West Systems brand at a typical store that handles it. West Systems is one of the highest quality epoxies you can use for building and has been used to build many a high power model...and some lower power too! West Ssytems is a thinner based resin making it ideal for glassing.
It is mixed in two parts and although it is not mixed in equal halves as other Epoxy Resin based adhesives, it is still Epoxy based resin, NOT polyester. For future reference, stay away from the polyester resins like those sold at Lowes, Walmart or other stores.
The base for West Systems is the 105 Resin. This is the only number available for resin and different Hardeners are mixed with it for different applications. I use the 205 Hardener pretty much for everything, even glassing large body tubes. I also keep some 206 and 207 Hardener on hand and have used them at times, although the 205 pretty much can do it all. Note that West Systems also makes a great adhesive for assembling a rocket as it's thinner consistency allows it to penetrate and soak in wood bulkheads and centering rings better than standard Epoxy glue. I use it to assemble the entire airframe, then use it again for glassing the same airframe.
You will need to purchase a pump system for West Systems to make sure you mix it correctly. The pump set used for 205 and 206 hardener is a "5 to 1" pump system, while the one for 207 and 209 hardener is a "3 to 1" pump system; make sure you get the correct pumps. Also, one pump from the resin and hardener will mix about 20cc of glue, or about 2/3 full of a mixing cup. for smaller ammounts, you need to fill to the gradient on the side of your cup in smaller quantites. I have done 10cc's for resin and twocc's for hardener (5 to 1) and it works fine. About the only downside to West Systems is that you do waste more, but cost-wise it is much cheaper in volume compared to hobby epoxies.
I hope this cleared it up a bit and wish you luck with your glassing. Ask again if you have any other questions...there are many of us here that can help.
Carl