There is a co-op in Castlebay, a shop almost on the route in Eriskay, and co-ops in Daliburgh and Creagorry, and I think there's still a little shop in Howmore, in between.
Once you are past Creagorry there isn't anything until Lochmaddy, and a shop and café in Berneray about 1/4 mile from the ferry (incidentally the new bunkhouse should be open by then, also near the ferry.) Bear in mind though that nothing is open on Sunday once you are north of Benbecula.
Further north, Leverburgh and Tarbert have reasonable stores and I think there's a little shop in Balallen.
There are hotels/pubs in Castlebay, Eriskay and Daliburgh, one in Creagorry, and one about a mile off to the west in Liniclate, a hotel in Carinish, another you go past at Langass but it's a bit pricey, Lochmaddy has a hotel and a café in the Arts Centre, and Berneray shop should be up and running for evening meals by then.
Leverburgh has a restaurant and the butty bus, I'm not sure how late the butty bus stays open, but they do good fish and chips on Fridays. Tarbert has a couple of hotels and a chippy, then there are a couple of cafes on the way up to Stornoway which I think are on the route, but Snoopdawg might have more up to date info on those.
The main trick when travelling through the islands is to make sure you are south of Grimsay on a Sunday.