Leonie, 100 grit is far to coarse, generally you can get up to 400 grit in most hardware stores, you probably want to get out a have a look at other stores or do a ring round, however at this stage I would persist with your original suggestion. You might just give the sander one last call and ask him what was the final grit he used, should help you determine what paper to buy, and remember to go through the grades only you will probably be starting with one of the higher grades anyway.