I also used soda bottle safety rings. And it actually was pretty easy to collect them the first time I saw them being used-- it was at the second PAX East. Some DMs were using them as they played 4E and right then and there I wanted a bunch. And fortunately... I had an entire convention full of recycling boxes filled with 20 oz soda bottles in all manner of colors. I acquired probably a half-dozen rings in like five different colors, all absolutely free.
You can achieve the same sort of thing pretty easily by going to your local pizza place that has seating-- they probably have a recycling box for soda bottles as well, and you can spend a bunch of minutes just pulling the rings off all the bottles (you probably want to ask/alert the employees first). I went the Pepsico. route, as they seemed to have the most varied color options of the different companies (and I'm one of those people who want them all to be the same size and weight, so I refused to also use other soda companies or milk rings.)
I don't use them for 5E much anymore (there are fewer conditions out there as often, especially now that "bloodied" is no longer a thing), but I still have my supply if/when I ever need them. And they worked wonderfully.