Yes, I did the "date your co-worker" thing. Bad, bad, bad idea. Not because there was a wretched break-up, but because it's just all around bad. Either, you keep it secret and have to do the stress-dance of keeping it secret - not good for any relationship - or you tell people which leads to a multitude of weird interactions with other co-workers. There is the constant nosiness for updates, possible jealousy, or accusations of unfairness. The biggest problem for me was that it just didn't work for either of us. We ended on fairly good terms - or so I thought. Then I started seeing someone else (non-colleague) and there was some seriously difficult moments at work because of the jealousy. Even if the person is in a different department or works a different shift, it can still backfire. My recommendation is do your spouse shopping elsewhere and treat co-workers like siblings or first cousins. We all have relatives that are really nice and even some that are attractive, but that doesn't mean you date them.
At the same time, former co-workers are fair game. I will also say this, if you are already married and both want to work for the same company, this can be successful. Marriage changes everything as the need to hide the relationship is gone and there is less needling from co-workers. Although even if married, I would suggest trying to stay in different departments with different bosses, for the sake of your marriage more than for the sake of your job.
I worked in Human Relations for 6 years so I am not only speaking from my personal experience, but also from the perspective of having to deal with real issues that can arise from romantic relationships at work.
So where do you look? Here are just a few ideas… Friends of friends. Lots of people have single friends looking for "someone." Don't do a blind date, do instead a set-up, i.e. a group goes to xyz event and you happen to sit next to the other single person. It's a great way to meet someone you already know a little about. Pick a hobby and find a group to join. For guys, join a church group, book club, or charity with lots of ladies. For girls, try co-ed sports teams (softball, volleyball, bowling), fantasy sports leagues (ones that are organized not just the random ones online), or try charity events guys attend - like poker games. I met my husband through his brother, who I knew from college. When I found out he had an older single brother, I said "What gives?" and had him arrange for a "chance" meeting. We met 3 times in bigger social situations, and after the third time, he asked his brother for my number and we went a on a real date. I'm a very happy SAHM with 2 beautiful boys. My 3 married sisters all met their husbands through people they already knew, and all are now SAHM's.
So go get 'em...just not at work!