I'm not sure what you mean by REI not having "mail contact," or why you would want to contact them by mail. They have 3 or 4 ways to get in touch with them:
1. If you go to their website, REI.com, and go to the "contact us" page you can see they have "live help," which allows you to do live chat with one of their customer service reps.
2. They have email contact as well, with a promise to get back in one business day.
3. You can telephone their help number.
4. You can get the phone number of the store you are interested in, and call that store directly (ask for the bicycle department).
If you get in touch with REI, they can help you with sizing advice, and then let you know where the bikes that are in stock in your size are located in LA. I wouldn't be surprised if they would help you hold the bike(s) in your sizes at the store that is most convenient to you.