This is a little off-topic to the process, but more about supplies for the process.
I thought I could pick up some cheap, used, Grolsch bottles online, until I started looking at the prices. On Etsy and eBay, these used, empty bottles are going for almost the same price for a single bottle ($15.50 for a pair; $21 for three), as a new 4-pack ($7.20-9.50), with beer in them. When you add on the shipping, and the wait, it'd actually be cheaper for me to run out to the store, buy a 4-pack, and empty the beer, for the bottles. Is it that there's so much demand for these bottles, with more home brewers, that there's a shortage?
Where do most people go, for affordable, fermentation-strong, flip-tops?