A lot of my local petsmarts have the smallest containers on clearance for $1.97, probably the cheapest option than having some shipped.
Yes, I bought a few as buying the ones on clearance was cheaper than buying a bigger bottle. I also bought a 500ml bottle from Amazon to refill the small bottles because it's easier to measure drops out for smaller tanks.
Those small bottle 'dropper' caps will fit on the 500ml bottle, as well.