To clarify Unregistered's post...
It's true that HD doesn't have much listenership, but you might be able to lease one on the cheap. I'd try KRTU (Trinity University), Univision, Cox, or even Clear Channel. All those broadcasters have HD signals that may or may not be in use.
As for translators, a translator is a step below an FM station, in the sense that it is legal so long as you use another station as the primary signal to base it off of. If you do get ahold of a translator you will then have to find a primary station if you don't have one for it. You can use an existing FM, AM, or an HD signal... The trick is doing all this and staying in a low-cost budget.