Below is an incomplete list of resupply options; the critical ones are listed, plus a few others.
I personally only had 1 official resupply point, at Hveravellir. I snacked up at some of the other locations, some of which were a surprise to me.
Karahnjukar Dam. On the east side of the dam there is a café on the south side of the road that sells hot sandwiches, drinks, and a small assortment of snacks (candy bars, cookies, and chips).
Nyidalur. You can send a package to this hut via the TREX bus line.
Hrauneyjar. This is a remote outpost with bathrooms, a nice café/restaurant, and a small selection of snacks like candy bars, chocolate, cookies, and chips. Their grocery selection is about twice as big as the café at Karahnjukar Dam.
Hveravellir. You can send a package to this hut via the SBA bus line. Mark the package “Hveravellir, Kjölur Higland Road.” Also put your name and estimated arrival on it.
Stadur. This small town is accessible via the Ring Rd after you cross Tvidaegra; hitch north. The first gas station has a grill, decent selection of snacks, bathrooms, and a payphone.
Landmannalaugar. There are two green buses parked at the fringe of the campground where you can get some hot food and a limited number of snacks.
Porsmork. There is a very small store here; go find the warden’s son so he can open it.
Skogar. Nearby there is an expensive hotel and two expensive restaurants.