I'd be surprised if a large portion of the community didn't feel the same way as you do. Especially since this is D&D where the concept of the GM is so fundamental. Maybe have a running tally and set up goals for the tip jar and what happens when each goal is met. Sort of a twisted version of crowdfunding.
GM tip jar in the cash shop. The community will be happy to pay for you to hire active GMs. And as they see the effect their donations make - whether it be through bot and goldspammer bans (make sure you announce bans in public channel) or GM led events - they will be happy to make repeated donations.