BLT's products are marketed in the USA by a company called Bicycle Technologies International. Here's a link to the
BTI Store Locator.
If you put in your zipcode it shows local stores that carry their products. I used a Dallas zipcode and found 20 cycling stores. 20! I only have 5 within 40 k of my house. The BTI site shows REI as one of their distributors but I couldn't find any of the products on their site.
Good luck findding it. If you don't have any luck, one of my local stores, Rapid City Cycle, carries the tail light. He's online as
Bike Canada. He hasn't updated his website to show the Flare's but I can't imagine he'd have a problem selling them online. Let me know if you still need it and I'll check with him.