Fostering cat lost. Chipped but not registered. Where to register?

I'm fostering a cat, "Rambo". Rambo was microchipped but the owner did not register the chip with any chip registration company.

Rambo is 8 years old and has been missing for 2 days since I took him in. He was in my house for about 1.5 weeks and was doing great and very friendly. He was going in-and-out of my own cat's pet door and just hanging around the front of the condo. Rambo grew up as an indoor cat, and started checking out the outdoors when he learned to use my cat's pet door. I saw him go out the pet door on Saturday night around 9pm. It's now Tuesday morning and he hasn't returned. I know from experience that all my previous cats I've had will go on 3-5 day camping trips and come back like no-big-deal. My only worry with Rambo is that he didn't grow up as a part-time outdoor cat, like my other cats.

I want to send out an alert notification to vets, pet stores, rescues in case someone finds Rambo.

Which microchip registration site do you recommend?

I have my own cat with HomeAgain, but if there's another service that's better, please let me know. We are located in Northern Virginia.

Thanks,

Check with these folks.  It looks like they cover ALL bases.  You may want to call them for more details.  The phone number is on the web site.

http://petkey.org/

Thanks for the info on petkey.

FYI, Rambo was found 3 condo buildings away from mine by one of my neighbors. He was just hanging out, probably someone was feeding him. When I brought him home, he wasn't very hungry and just walked around like what-was-all-the-fuss.

I'm so glad Rambo is home safely.  I guess he's pretty laid back and makes friends easily, which is a good thing.  Most cats would go into hiding for a week or more before coming out and begging for food.