http://www2.mailordercentral.com/quantummodels is really big, and has decent prices.

http://www.hobbybarn.com is the only other place I've ordered besides tower. Mom&Pop shop, but nobody could beat them on Irvine engines, and I was very happy with the
transaction.

http://www.tti-us.com/sim/shop.html is a great place for radios, buddy boxes, sims, and sim interface cables. Got some nice stuff from him and he was very helpful when I had problems.

There's a lot more, but I haven't ordered from them. http://www.hobbico.com has a lot of stuff. http://www.horizonhobby.com has a lot of stuff and is the home of Hangar9 - Joe