There are several national programs and agencies you can go through if you are interested in adopting a mustang. There are also quite a few private organizations that specialize in mustang adoption as well. Here are some resources to get you started!
National Wild Horse and Burro Program

You can adopt a mustang or burro directly through the U.S. Bureau of Land Management. There are holding facilities/adoption sites in several states. These animals are wild and untrained. The Adoption Incentive Program provides up to $1,000 to adopt an untrained wild horse or burro from the BLM.
BLM Internet Adoption Program

Not close to a holding facility? You can still adopt through the Bureau of Land Management's Internet Adoption Program. Same rules apply as in person, and bidding begins at $125. The BLM also periodically brings their mustangs on the road to eastern states for adoption.
Mustang Heritage Foundation

Home of the famous Extreme Mustang Makeover and Trainer Incentive Program! The Mustang Heritage Foundation is dedicated to facilitating successful mustang adoptions around the country. They have also created the National Wild Horse Adoption Day, which coordinates events throughout the county.
Northern Nevada Correctional Center Saddle Horse Training Program

This famous program takes mustangs from holding facilities and pairs them with inmates for 120 days of gentling and training. The saddle-trained horses are available at auction several times a year. Bidding starts at $150.