eBay is the go-to place for older processors............ craigslist, offerup, and possibly letgo are worth a look because sometimes you might find someone who is selling a CPU for a good price.
High end processors from yesteryear like the Core 2 Quad q6600 and AMD Athlon 64 x2 6000+ can be found on ebay for about $5.
However, it's damned near impossible to find a Brand New In Box processor that's several years or even a few years old. So you're going to have to settle for a used one.
Which brings me to a point....... The CPU is the last part to die in a PC. A well cooled CPU, even if it's running 24/7 can last for decades. Even gently overclocked CPU's will show no signs of degradation after being overclocked for several years, provided the user knows how to overclock and understands the voltage limits. More often than not, the signs of a bad CPU are the same as the signs for a bad motherboard. The only way to know for sure is to replace either part. In these situations 9 times out of 10 it's the motherboard that is broken.