The usual suspects - snapnames & pool are probably still your best bet.
The .com drop market is a little more mature than here in the UK and a lot of the main registrars now send their dropping domains to auction rather than let them drop as they once used to.
tdnam for godaddy
namejet for NSi and enom
To name but a couple.
The .com drop market is fierce and their are quite a number of domainers and orgs with deep pockets, paying (imho) way over the odds for some average domains.
I tried, albeit unsuccessfully recently, to nab adpal.com in an auction at namejet but it ended up too rich for my blood.