Forum Moderators: skibum
Since you are running the show, you could survey your users and ask them which shirts they would be interested in, then find out how many people would order the winning designs, then take pre-orders to pay for the out-of-pocket expenses on the order.
Is there any webcomic author forums that talk about monetization? If so I would check those out and read what others are doing and how they have struggled or succeeded.