On becoming a wet nurse vs. just simply donating milk regularly, I'd say some things would be:
-Setting expectations with the family you choose to work with. How long are they wanting this service of you, vs. how long you'd like to offer it.
-Decide what you'd like your compensation to be, if any.
-Making sure you're willing to commit to the tribulations of breastfeeding. If you'd actually be directly nursing the child, making sure you're prepared for the physical challenges that often come with that. And if you'll be a dedicated pumper, making sure you're prepared to take on the responsibilities associated with nursing a child (watching alcohol intake, eating well, not taking medications, etc.).
As for where to advertise, I'm fairly certain that the sale of raw milk is prohibited in most places... That said, Craigslist, any natural parenting groups, your tribal area on MDC, or any local adoption groups might be good places to start.
To me it seems like a very complex relationship to have with strangers. Once money exchanges hands, it can change expectations for both parties, so I would make sure that you're very clear about what your requirements are for an arrangement like this.
Best of luck!
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