I had forgotten about those... These target hit indicators work really well and you can see hits even in broad daylight.
It can also be used on hanging steel but, if the wind is strong enough to move target, you might get a false-positive. For this reason, using it on static targets works best.
I have one that I made myself for under $40 (the most expensive part was the flash). All that is needed is a mercury tilt switch (~$3 from Amazon), a flash with a PC connector (I paid $25 for mine but I am sure you can get them even cheaper on eBay) and a PC cable ($$$ depends on the length; you can find them for less than $10).