There are quite a few plants that make good warm-season cover crops, but the one that would be best depends on your goal in using them.

If you want to add a LOT of green manure to your soil AND attract beneficial insects AND still be able to plant a crop in mid- to late-summer for a fall harvest, then buckwheat would be a great choice. It is a non-legume broadleaf, so it does not require an innoculant. It DOES NOT fix nitrogen in the soil if that is your goal, but it does a wonderful job of smothering summer weeds. It also accumulates insoluable phosphorus and, then, when you turn it under and it decomposes, it releases that phosphorus in a form that plants can absorb. It is a very fast grower so it is a great cover crop to use in between crops. It will mature in roughly 30 to 45 days. Depending on the fertility of your soil and the amount of rain that is falling, it can get anywhere from 8" to 3' tall.

If you are looking for a legume that will fix nitrogen (and you'll need an innoculant, obviously) there are a couple of good choices. The old standard that has been used forever is cowpeas or field peas. You also can use snap beans, and I'd use bush beans if I was going to till then under using a rototiller. If you have a tractor, you could use pole type beans (just let them crawl and sprawl) and turn them under when they are done.

If you want, you can plant both buckwheat and cowpeas in a mix that is roughly 70% buckwheat and 30% cowpeas or, if your soil really needs a lot of nitrogen, do a 50/50 mix. Use an innoculant with the cowpeas but not the buckwheat.

There are several clovers that make good cover crops, but you have to have pH of 6 or 7 to use them, I think.

As far as where to find them, check with the local farm supply store or feed store or call your county's agriculture extension office and ask them to recommend a source.

I like to use amaranths as cover crops that double as ornamentals. Amaranth is less commonly used here. I use it as a dual purpose plant. I grow several different kinds that produce different colored flowers/seedheads (red, orange, yellow, green). I harvest the flowers or seedheads for autumn decorations and then use the plants for green manure. The only thing about the amaranths being allowed to set seed is that they reseed quite prolifically, so if that would be a problem, you need to turn them under before they set seed. I only use a small amount of them in any given year, so I get my seed from Seeds of Change in New Mexico.

Finally, I've linked some info on using cover crops in Oklahoma.

Dawn

Here is a link that might be useful: Oklahoma Cover Crop Info

Holman Seed Farms in Collinsville (N of Tulsa) is the place I purchased some cool-season seed this past fall, after a lot of research and price-checking. I was getting 50lb bags for coverage over multiple acres.

Their number is 918.371.2195 if you want to check on what they might have for summer crops.