Trade a mailing with another list owner.
They mail for your list. And you mail for theirs. No money has to be exchanged. You don’t need to be an affiliate.
Simply find other publishers with list sizes comparable to yours…and mail for one another.
Instead of promoting a product, promote the freebie they get for signing up for your list. I have a couple of clients that used this as one of their primary list building methods. They built up a basic list (of a few hundred) and then immediately set to growing their list by finding others to cross-promote. They’re growing their lists by hundreds every week (at zero cost).