Xisca, you need to go visit the
Desert Legume Project. They have quite an extensive seed bank of just what you are looking for, and they will send you samples (a couple dozen seeds) to try out. Although they are affiliated with the University of Arizona in Tucson, they collect from desert areas worldwide. They are a non-profit, so your order will come with an envelope so you can make a small donation.
In addition to acacias, you should consider palo verdes (
Parkinsonia spp.) as well. They are considered a noxious weed in Australia, where they have eradication programs, but they are endangered in the Horn of Africa (goats and camels really seem to like them).