Define "good" and "cheap."
Check your local chain sporting goods store; Big 5 is what we have here, and the three basic Mosins go on sale for $89 all the time. They also carry Turkish and Yugo Mausers and (up until a while ago, at least ) K-31s.
There are several sites specializing in surplus firearms, some I can think of off the top of my head are:
classicarms.us
empirearms.com
southernohiogun.com
samcoglobal.com
aimsurplus.com
Places like centerfiresystems.com and jgsales.com sell a bunch of everything, but do have surplus rifles. Empire sells high-dollar, high-quality collectible rifles.
Mosins are pretty much universal these days since they can be wholesaled for so cheap - M91/30s, for example, can be bought for around $69 online, but then you have to factor in shipping and FFL transfer (unless you have a C&R license).
Speaking of which - if you're looking to start a collection, get yourself a type 03 FFL (aka C&R). You should just about recover the cost of the license ($30) on your first C&R gun assuming your local places charge $25 for transfers like mine do.