I used https://sppare.me/ and the process was super simple. You can order kits for 1, 2, or 3 samples. They all come with a year of storage and can be paid in installments, then storage $245 ($165 for one sample) per year. Each sample gets 3-6 vials of sperm, which is about how much it takes for a successful pregnancy from IUI.
You don't need any referrals or anything, just place the order and get the kits in the mail. They recommend a week of abstinence before each sample to get the highest sperm count. You collect the sample, package it up, and bring it to a FedEx dropbox.
I don't have any experience with retrieval yet, but the collection process was really easy and I didn't have any issues. The hardest part was finding the FedEx dropbox to bring it to.