Duotope is a good site, especially if you're just starting out. If you have a twitter, you can add them and get all the new calls for free, but it's kinda clunky to do it that way. I'd suggest joining, at least temporarily to check it out, see if it works for you--you don't have to stay, and you can always make a list to take away with you.

I made a list of all the likeliest markets back when the site was free, and filed them away with links, so then it's up to me to do the rounds once a month or so (well, more like once every week or two, because different markets post their calls at different times).