Seasonal Retail Shopping, with Science and Mystery.

 It's funny to me that crazy people, especially people that might otherwise seem sane but have one or two beliefs way outside the norm are still often referred to as the "tinfoil helmet crowd". It's been decades since there's been actual tin foil on retail shelves. Aluminum supplanted it years ago. It imparts less metal flavor to the things you wrap up in it, and aluminum became more available than tin, bringing prices in line for consumers.

Often, this is how technology works. An old thing gets taken over by the new thing, and most of the features that everyone wanted were covered. The other side of that is if you wanted one of the features that were available before and aren't now, too bad. In this particular case of Halloween, it's the "party bulb".

 Most single color bulbs started as incandescent with colored glass bulbs, of varying quality. It's inherently inefficient to start with low quality broad spectrum incandescent light and then filter out big chunks of it. Red, orange, and yellow were easier to do than blue, green, or purple, as standard filaments produce more of those warmer colors.  LED single color bulbs were much better as they were better at very specific wavelengths and thereby didn't need colored glass lenses - they could be done with traditional clear or frosted lenses. But, LEDs are able to be even more complex now, and the bulbs are color changing. You can now get one bulb that can do the whole gamut from red to purple, so you just tell the bulb what color to emit either from your phone or from a little controller.  This makes red, pink, orange, amber, yellow, green, blue, and purple single color party bulbs - even in LED - obsolete. What gets left behind?  Black Light / UV bulbs.

That's not to say that you can't get them, but over half the places that might stock the 'party bulbs' before are just getting color changing bulbs now, and they might sacrifice what few UV sales they were making before just for simplicity in ordering.

Sometimes at Halloween, you may see incandescent 'black light bulbs', and there's always been fluorescent UV bulbs for the serious black light poster aficionados. LED technology has added a couple of other choices to the mix as well. Here's how it shakes out. (Wavelength given in nm - nanometers.)

Black Light Incandescent - doesn't have a lot of actual UV output, but some. Most of the output is in the 400-525nm range, mostly purple visible light. What UV output there is tails off to nothing below about 370nm. (This was new to me, I did not realize the incandescent bulbs were as ineffective as this.)

UV Fluorescent - peaks at 370nm, with a normal distribution between 330 and 410nm. This is the standard we expect. 

UV LED - comes in two flavors, one that peaks at 365nm (more expensive) and one that peaks around  385 - 395nm. Unsurprisingly, the less expensive LED version is more like the incandescent version and emits a lot of purple visible light.

After a little digging around my local retail stores, I found what turned out to be my only real choice, a 2' fluorescent black light. It wasn't super expensive (under $15) but it had an extra feature that I wasn't expecting.

EVERY. SINGLE. PACKAGE. HAD. BEEN. OPENED.

To me, this is always a bad sign.
If all the packages are open but nothing is missing, that makes me think that either one person was looking for something else or trying to figure out what they thought was included but isn't actually included.