Very good question, and the answer is: it depends.

Most seminaries that I know of have a list of men available for supply preaching. At the seminary I attended, there was actually a sign-up process where we let the Student Services office (who handled it there) know which Sundays we would be available for. Then, when a church in the general area (which was sometimes not really in the area– a couple of times I drove more than four hours!) need a supply preacher, they call the seminary. At my seminary, you had to have completed at least three of the four homiletics courses to get on the list.

If your seminary doesn’t have such a list– or if you’re not eligible– then you’ll have to find other creative ways. It might, in fact, be as simple as calling up a pastor and letting him know you’re available. If so, here’s my advice: choose the smaller churches to start with. No, not because you want to “work your way up” as you get better. It’s because the larger churches will have other staff or interns who will be the default supply preachers when the Senior Pastor is away. Smaller churches, though, won’t have such resources, and will be glad to know of an available preacher during the Pastor’s vacation or in an emergency.

Your best bet, though, is probably your seminary. Even if you’ve graduated, they may allow you to stay on their list. And if you’re especially experienced (maybe you worked in ministry before attending seminary) then you might be able to talk them into letting you get on the list before the normal eligibility requirements are met.