Mikey Garcia -- the king at 140 -- says he's willing to go up in weight for a shot at a super fight with Errol Spence ... and he wants it bad!
The reigning IBF light welterweight champ tells TMZ Sports he's already trying to set up fights AFTER his lightweight brawl with Robert Easter on July 28 ... and he's got his sights set on Spence.
"I'd really like an opportunity to fight Errol Spence," Garcia said ... "Let's get together after this fight.