Where do you get a dog euthanized, and how much $?

I have a 16 year old Chihuahua and his hips are getting so bad he can barley walk. he poops and pees all over himself too. I hate to do it but I think It would be best if I put him down. but I've never done this before and I don't know where to go to do it. or how much it cost's. (mesa, AZ) this dog has been my buddy and body guard for a long time so please be nice.