Why so particular in a specific brand name.. what are you needing to achieve?? UMAXX is the name that Koch industries (?) trademarked about 20+ years ago to name their stabilized urea that uses DCD & NBPT to act as urease & nitrification inhibitors. They also marketed the UFLEXX product which contained a lesser amount of these inhibitors, essentially cutting the duration of release from 8-10 weeks to 6+ weeks (which is what is more common in the LCO field anyhow being that they tend to run on 6 week application cycles). These same products (DCD & NBPT) are used by many other manufacturers in different strengths and amounts so you can find alternatives depending on what your supplier carries, but asking for UMAXX is akin to being specific about Kleenex as a tissue brand. I don't mean to diminish your question, just wanting to make sure you're not following some advice about such and such being thebomb.com There are different products to accomplish the same thing.
If you're set on finding it try suppliers that carry JR Simplot products. I think they're the ones who have that name now. Ewing Irrigation is someone that carries that line but they tend to stick to UFLEXX, from a cost & practicality perspective (I add practicality because nitrogen for turf grass is not buried under soil as in farming, so UV rays and surface disturbances will cut the duration of any stabilized or slow release nitrogen, to a degree. Most people will choose value over some theoretical advantage that may or may not translate to the real world). I just looked up what Ewing sells their 46-0-0 UFLEXX and it's approx half the price of UMAXX, for 2-3 weeks longer release curve.
Check out Simplot's website filtered by available UMAXX products
. You'll see that UMAXX is also available in different fertilizer mixes not just 46-0-0, and if you click on the Label for any of those products and read the Derived From notes you'll see exactly what I'm referring to. Here's a snapshot for ease of viewing.
I guess the question is, what are you needing to achieve? There are other forms of controlling nitrogen that results in even longer release durations, if that's what your objective is. Here's a quick table for reference. Hope this info helps.