Basically, there are no more k6-III processors available, although Spartan Technologies at
http://www.spartantech.com/ is currently listing 400 mhz k6-III processors for $142. I don't know if these are 2.2 or 2.4 volt processors. I bought one of the 2.4 volt units last year for $65 and was able to stably overclock it to 448 mhz (4x112) at 2.8 volts.
I was fortunate enough to buy a genuine 500 mhz k6-III a few months ago from the same company for $199. This is a beautiful chip. It runs completely stable at 504 mhz (4.5x112) at 2.2 volts, proving it is a genuine 500 mhz processor. As you probably know, 450 mhz processors have to be pushed to 2.8+ volts to reach 500 mhz. k6-III processors just cannot be overclocked to any great extent.
It's too bad AMD never released the k6-III+ (.18 micron versus .25 micron), because that would be a killer CPU and quite overclockable. Of course, the real reason AMD never released this beast is because it would have blown the doors off the pre-thunderbird athlon and the pre-coppermine pentium III. Clock for clock the k6-III was simply the fastest processor around until the thunderbird and coppermine processors were released. At 504 mhz my k6-III scores 52+ on CPUmark 99. Check out what "classic" pentium III and athlon processors score at 500 mhz. My 448 mhz CPU posted one of the fastest DOOM benchmarks ever using an ATI All-in-Wonder Pro!!! You can check the DOOM benchmark page at
http://www.complang.tuwien.ac.at/misc/doombench.html.
At this point, your best bet to upgrade a socket 7 motherboard is the k6-2+. AMD has only released this CPU as a mobile version, and it only has 128k level 2 cache, compared to 256k for the k6-III. But it is a .18 micron CPU and apparently very overclockable. There are some issues about which motherboards will support this processor. Basically, you need a motherboard which can set the voltage to 1.8 volts, or at least 2.0 volts; and your BIOS needs to recognize the k6-2+ processor.
There are in-depth articles on Ace's Hardware at
http://www.aceshardware.com/ and Tom's Hardware at
http://www.tomshardware.com/ about the k6-2+. Vega Computer Industries at
http://www.vegapc.com/ is currently selling 500 mhz k6-2+ processors for $89. This CPU should easily overclock to 600+ mhz, making it comparable to a 500 mhz k6-III.