Hey there!
This is actually not allowed in any of the boards here. The main reason is people will start sending them out to the ones who sign frequently since they're "easy" successes. I'm not implying that's what you're doing, but that's the major concern with this.
I suggest sending to the Infinity War cast at the via venue address. Search on the search bar a celebrity like Dave Bautista (I'm actually sending to him ASAP) and the Infinity War address SHOULD be there. It apparently doesn't stop shooting until November.
Other people you'll either just have to take a blind shot in the dark with the address in the database or hopefully when you search the person, somebody else has already had a success.
I'm big on sports autographs as well so I get why you'd branch off. It's nice to switch it up and get something different. My advice is to send it to whoever you like no matter if there is no success posted. Someone might send it and not get a success, but you could send a photo or two and get them back signed. It's all about luck and if they liked your letter enough.
Good luck and I hope to see you post some MCU successes, especially since I might be starting a collection soon
- Cody