Our designer works on an hourly rate. You put down a retainer for the first batch of hours (10-20) and then you can switch to paying for what you use in a given month.
You can search for a designer here:
https://www.asid.org/referral-service
Also, I think local designers participate in a Spring Shape Up -- you can hire one for four hours or so to get help with something specific. If you liked that designer, you could hire her or him for more.