Those look like either T-18 or T-25 staples, I find that most saddles use a T-50 size. I purchase 1/4" Stainless Steel T-50 at Home Depot.

These are the shortest size made and the stainless steel version helps prevent rust-out. We use a hand-held T50 Arrow stapler for recovering saddles. We do about 25-30 a year.