I have barn swallows which are just as good as purple martins. In fact, the fact that I have barn swallows may be the very reason that the purple martins refuse to colonize here.
My research indicates that purple martins require a clean, apartment style bird house, or a series of hollow gourds hung on a pole. The bird house is mounted on a sliding pole that can be raised and lowered. The bird house has removable floors so that it can be cleaned out each winter. You should take the bird house down over the winter, or (at least) remove the floors in the compartments. Then, you have to watch for the "scout" martin in the spring. You immediately hoist the clean bird house up a pole (with the floors re-installed) and hope the "scout" bird spots it. He will then bring the rest of the flock back to move in. The trick is to get the martins in before sparrows move in. The two species will not live together.
I can't vouch for these instructions because they never worked for me. In contrast, you only have to leave your barn door open, or a window, to attract barn swallows; and supply open space to fly and a lot of insects for them to catch. They leave in the fall as quickly as they appeared in the spring. That's why barn swallows are easier!
Martins do want a water source but what we and others did was put out stock tanks filled with water or large bird baths. They don’t need a lake or a river just a source of water and it should be large enough for the number of birds. Several bird baths also worked. I know the town where I grew up and this was started had no body of water for many miles.
I’ve always liked barn swallows. I guess they will only take up residence in a barn/outdoor structure of sorts with easy escape routes. I had to laugh with everything about the Purple Martin... seems like they have the humans trained real well. LOL! Fickle little things, aren’t they?