Becca - though I've not taken advantage of the process, I believe you should be able to seek child support through your local court offices.  At least in my local district court, people can seek child support free of charge.  You'll probably just need some basic information of your ex.  I'm not as familiar with OK guidelines, but I'm pretty sure they do things the same as we do in KS.  This means that you really don't need to contact an attorney to get child support started.  It will be based purely on what both of you currently make.  Are you currently working?  Oh, almost forgot, you may consider a temporary custody order too.  This leaves you as the primary custodian and will leave you in charge of everything.  Usually these kinds of orders are followed up by a permanent custody order though, which can end up costing you quite a bit.

My advice would be to simply start child support and document exactly how long you remain as the primary custodian.  The court will probably identify you as the primary custodian anyway.