My experience is CUs are worse than banks for appraising. They want to be real cheap, and the one I worked with dropped me fast when my MV was less than what borrower was expecting on a refi.
Other than that, treat like a regular brick and mortar bank. Just go in, talk to mortgage lender or branch manager. Explain quickly why you are there (no false pretenses).
I've always had as much CU work as I wanted, which was little because their fees were always low in my area.
Many use a CUSO for underwriting and appraisal ordering as well. The midwest uses TUAS as lot, not sure if they get out west or not.