You have to check a variety of sources 2-3 times per day. The ones I used to check were: ebay, craigslist (especially California), boogieboard, harmony central, gear page, petrucci forums, guitarcenter used section, and other popular guitar/amp forums. Even google works well. Then just jump fast.
Yes, you have to keep your eyes wide open. I purchased my 84 combo right here on the forum and my 85 head on ebay. Also, becareful it seems not all sellers realize the difference between a true C+ and a modded C. I have seen a number of modded C's being advertised as a C+, but remember the serial number is the only way to be sure.