Any reputable tire dealer or store can get them but most will try and sway you to purchase the ST tire that gives them the largest profit margin which of course they have in stock. I have several good friends that are mechanics, a couple work for major dealerships and one works for a tiny garage/gas station, either can get any tire including Maxxis within a couple of days.
I went the online route when we purchased our current set of Maxxis ST tires over 4 years ago and used DiscountTireDirect.com as suggested above. They were delivered to my front door 3 days later for free via UPS. Took them to our local Discount Tire and had them mounted and balanced and warrantied.
Last year I wanted to replace our spare with a Maxxis and tire dealer once again try to sway me to another brand as they didn't have Maxxis in stock. I told him I guess I have to go online and started walking out the store, amazingly he stated he could attain a Maxxis tire by the next day. Sure enough he called by 9:00am and we had the Maxxis mounted on our spare an hour later.