I usually fill up my alcohol bottle before my trips, or bring a canister that weighs out to more than my calculated need for my trips. But I haven't ever had to refill while "away", such on a thru hike.
I know you can buy denatured alcohol at hardware stores, but it usually comes in 32oz cans. Heet comes in 12oz bottles. And 91% isopropyl alcohol comes in 16oz bottles at the pharmacy. I only have a 250ml fuel bottle, which is about 8.5oz. Assuming you have a 12 or even 16oz bottle for fuel, say you roll into a town with a few ounces left in your gear (12oz would last me about 6 days and I am calculating resupplying every 3-5 days). Seems like whatever you buy, you have extra fuel. So you'd likely just, what?... pour the extra down a toilet?
Preemptive second question:
Why don't people just carry their alcohol in the bottle it comes in (like the HEET bottle?)