Definitely look on eBay or in your local camera shop, if you have one, for M42 lenses. I've been able to find used ones rather inexpensively, since they were somewhat of an SLR lens mount for a couple of decades and they were made by many different makers. And as Ian said, the Takumar SMC (Super Multi Coated) lenses are especially nice.

Before last year, we had a large antique mall in town that had a couple of booths filled with cameras, most of them junk, and a few lenses. I had mixed luck with the cameras I scavenged there, but it was easy to identify the M42 lenses because they're screw mount, instead of bayonets that all look the same to the untrained eye. In our local camera shop, I've been able to get M42s rather cheap because it's the Nikon, Canon FD, and Pentax K-mount lenses that fetch the higher prices.