Encourage your students to purchase from the store. It's okay to mention other stores when the store you're teaching in does not carry a specific item.
Please, don't compare the store unfavorably to other stores in the area during the class. I took a class recently where the teacher kept praising a nearby store and denigrating the one we were in. The staff person on duty was really uncomfortable, and so was I.
Also, following up on one of Sher's comments, if you spend time in the store, you'll be familiar with what they sell. Then if your students will need items not for sale at the store, you can either make arrangements for the store to get them in, or plan to bring them yourself.
Carole