Can they be grown in mid OK? How do I find out more about them?
I recommend the "Learn" portion of the Dixondale Farms website for onion education and plants.
There is a map on the site that shows Oklahoma in an over-lapping area of both short and intermediate types. I live very near the top of that area so I grow only Intermediate Day types and I grow mostly Candy, but there are several choices.
It is possible that the sweet Texas onion you speak of is a 1015Y Texas Super Sweet which is a yellow onion that is frequently planted in Texas on Oct 15th, hence the name. It is a short day onion variety. Onions are day-length-sensitive, so you will grow better onions if you understand what that means and choose a variety that works in your climate.
Here is a link that might be useful: Onions
I think the Texas 1015 is the one I'm looking for. Where do I get them?
I live in Mcloud, but i work on the west side of okc - the other day I stopped into the the Bethany country store on Rockwell, and bought a set of those texas supersweet 1015.
We don't know where you are, so it's hard to recommend a place. You might try calling K&K nursery in Norman, Horn Seed in OKC, or maybe even Atwood's. It's a popular variety so it shouldn't be hard to track down locally.
I bought onions at the farmers co-op, so they should be pretty wide spread by now. I had to buy Bonnie plants because the place that sells Dixiondale is not open yet. I plan to buy Dixiondale also if I can find them this year, they seem to do better for me.