If i were you i would be contacting fighters directly via twitter and Facebook.
Wont take much time and I'm sure if you cover the costs of gloves (to sign) and postage etc that some fighters would happily help you out.
When I collected boxing memorabilia many moons ago I got address of managers/trainers from the BBBoC Year Book.
I sent loads of letters and got some good signed photos back. But I had a print of Naz so I rolled it up in a tube and put in tape and a envelope with my name on it and posted it off. Basically there was no fee for the boxer to pay, so it was pretty simple.
I also sent loads of letters to boxers in the States, but only got 1 reply that was from the Kronk Gym, I got a great letter from Tommy Hearns with a lovely signed photo.
Years later after I moved house I sold them all (had them all framed on my stairs wall) on Ebay to a pub 'darn sarf' for a good few quid.
So there is a little info on how to get some free pics.