About 4th and South St is a former Synagogue that is now a flea market. They are open on the weekends and have some MEGOs. I saw Sonny and Cher the last time I was there. They might be open during the week but I don't know for sure.
12th or 13th near Pine St. has a little old toy shop. I think it's called Peace....something. They have old toys. I don't have any exact addresses for you. The Synagogue is worth checking out. It's not on South St but a block off it. If you go to Johnny Rocket's or Jim's Steaks just ask those guys. It's a block from there.
Brave New World on 2nd right off of Market has Retro Mego figures. That's about the best you can do for Center City.
If a Mego figure of God did not exist, it would be necessary for EMCE to invent him.