I've personally never used any of the services, but if you are a Costco member, they are affiliated with IdentityGuard; $9.49/mo for regular members, $7.49/mo for Executive. I'm sure Sam's Club and BJ's have similar affiliations with other services, if you're a member of one of those. Other than that, just do a Google search for "credit monitoring" and you'll find a number of sites, but I can't recommend any one since I have no personal experience.

As for checking periodically, you can receive 1 free report from each of the 3 major credit bureaus once a year, by going to www.annualcreditreport.com - the federally-sponsored website. What I do is get one report every 4 months, from a different agency each time.

If you want to pay to check more frequently, those options are available. You can either pay the credit bureaus directly, or you can look for a 3rd-party service. Again, do a Google search for "credit report" and you'll find plenty... just look for ones that appear legit and heavily-visited, and you can probably find ratings for various sites if you search for that, too.

The benefit of a monitoring service is that they check your reports automatically every 24 hours, which means that you get alerted within a day of any suspicious activity (new account opening, etc.). If you're truly paranoid about identity theft, that's the way to go, because if you only check your report every few months, a lot of damage can occur during those months. (That said, I've never been a victim, myself, so this is only what I imagine could happen and what I would do if I were in your shoes.)