Flat staples? Wow, who would use them? Just about every staple gun made. All you need to do is measure the width across the top, the width of the staple itself, the leg lendth and visit Home Depot and see what brand comes the closest.
When I attached new rubber to the front fender splash shields I found the original staples were 0.047 inch dia, the staples supplied with the new rubber shield was 0.062 inch dia - unacceptable. So I made some from 0.047 inch dia safety wire using my arbor press and a homemade die, enough for a couple of cars.
Jim