I have a Darkbeam mini, costs about $10-12, it works great. I don’t know about the filter. I took mine to Belize and the trees light up like Christmas!

You can track down the article mentioned by Steve here:

https://www.californialichens.org/calsbulletin

Volume 32(2): Winter 2025

Notes on Alternate Tests for Atranorin and Usnic Acid and Using Longwave UV Lights to Determine Lichen Chemistry

Authors: Jason Hollinger pellaea@gmail.com; Ken Kellman kkellman@sbcglobal.net

In addition to the two mentioned above in the CALS bulletin, I can also vouch for the Consciot U20, $12.39 on Amazon. It is bigger and heavier than those two, but still a small flashlight size 6" long and 1 1/4" diameter. It is also rechargable with USB-C.

The visible light filter is critical to see the colors shown in the bulletin article. Those without the filter have a violet light that drowns out the fluorescence. 

The Consciot https://www.amazon.com/Consciot-Flashlight-Rechargeable-Ultraviolet-Blacklight/dp/B0CMHYCQH4 comes with a visible light filter already on the front glass
Most of the time the visible light filter is not an attachment but comes built in.

You can buy specific Uv pass visible light filters such as https://midopt.com/filters/bp365/
but make sure that the peak of the pass through is around 356nm