Commissions are something you find out folks be interested when you post up that specific commission ad. Also, even if you don't get a bite your first time, don't be discouraged. Sometimes you end up not reaching out to folks looking for commissions at the time, and you should try again a little bit later (Like after 2-3 days if no one takes a slot). Big thing to keep in mind is to make sure that people other than your watchers can find your commission announcement. So if you are on FA, I highly recommend using a Submission to announce commissions because FA journals are NOT searchable and only reach watchers when your target audience is actually "People looking to buy commissions". DA and Weasyl have searchabe Journals, so people can find a com announce through Journals on those sites.