I have a routine basically. The one I like to purchase one from is sportlot.com. The thing I like about that site is that the shipping costs are not so outrageous. There you purchase from one seller since sportlot is more of a listing site. Here is what I do:
Enter my search for Troy Aikman. Then I go through the list and find the first card I need (they are listed starting with the oldest in alphabetical order) Once I find one then I open up that sellers list and find every card that seller has that I need. I then do the same with that seller for my other two PCs Detmer, and Novacek. I found in the beginning I could get a ton of cards for cheap and am still working through them. It's not as professional as COMC because you are dealing with individual sellers. However, I did find that the mistakes made were honest and dealt with appropriately. I tried to do this but eventually the call of COMC got the best of me and I purchased every card of Aikman that was under $5 and every Detmer and Novacek card they had so I could get a big chunk.
After that I look and Ebay and then the sales here. If I find someone on here that has something I want for a reasonable price I purchase it (I have to admit part of it is that I want to get my posts up because at some point I would like to put all three of my PCs in for supercollector status). Ebay I can find decent cards but you pay a fortune in shipping and by far I have had the most issues on there. You see people on here always complaining about some buyers and some complaining about sellers so you are screwed either way. I have gotten damaged cards that were pretty rare and when I let the seller know I got the we always package our stuff properly, but if you aren't satisfied send it back for a refund reply. In the end I kept the item because I don't know when I would find another and it was the only one I had ever seen for sale. But I have a $30 POS now. But I'll take that over a blank spot in my collection.
The last option for me is going to the LCS. We just recently got one here where I live and I bought every Aikman card he had but I paid probably double what I could have gotten them for even on ebay. The trick they used is have someone who has no authority to make a deal run the store so either take it or leave it. I needed them so I purchased them.
I have also purchased on Beckett and would buy from there again once I got through sportlot.
I only wish there were more ways to purchase cards. I really should get all my extras organized and sell them to have more money to finish out my collections.'
If any one knows of other places to purchase from I would like to know. I'm up for trying out new sources.