Just go to Walgreens. I think they have them. Many Hardware stores, Target, maybe Walmart. Look for a good old fashoned
Black Light. That is what I have. I am not good at measurements but I would guess about 14 to 16". They are also good for checking other collectables and antiques to find hidden damages, alterations to artwork inks and printing... etc.

Also, you can check your pet supply stores. They have lights for spotting pet stains. They are simple battery operated blacklights. They work ok but are a little low power. I got one of those at Petsmart. Comes in handy at the bar also to check ID cards for the security markings that are put into the laminations of most Drivers Licenses. I find though that the full size plug in lights work better.