how do you get considered for cob?

I would say wait until spring, if all the sororities cut you during formal, they had their reasons for doing so and won't want to COB you. if you become friends with girls in sororities during the semester, they'll have a better opinion of you when you come back in the spring because people will know you personally and can vouch for you

I would let your friends that did pledge know you're thinking about COB and would like to hear if their chapters are doing it in fall or spring, and like @umm said, get to know people better. For spring every chapter is a little different in how they COB- some may only have a couple of spots because some seniors graduated early or a some dropped or transferred between semesters, and some may have more spots to fill to get up to total. If you have an open mind about where you go, get involved around campus and get to know actives and pledges this semester, you would have a reasonable shot in the spring. And if you keep your nose clean and your grades high, and and open mind, you can try formal again next year.

As for fall COB, it's more difficult because chapters are just trying to hit total after recruitment and not filling between semester drops or early grad spots. They usually look for girls they wanted but who dropped them during recruitment who then didn't get a bid (usually SIP'd a higher tier chapter) as their targets. But if you're open to it, it's not going to hurt you to let your friends who are pledging know you're interested if they hear anything.