I used OKCupid to find my girlfriend (or rather she used it to find me...) and I found it fantastic. Great options for gender and sexuality, option to hide from straight people!
I think in your specific situation, the best option is to let them know beforehand, but only when you're comfortable. It's not your obligation to tell them right away, and quite frankly, it's your choice, because you aren't lying about who you are. I never put that I'm trans on my profile for dating, I'd rather bring it up after getting a chance to talk to someone first. Best of luck!!