The finest grit sandpaper I've seen at the local Home Depot, or Lowes, is 600 grit. I do use 400 and 600 when I begin sanding, then only wet, and follow it up with finer grit pads. I tend to use 400 and 600 grit paper which has already seen service in sanding down the oak display bases which I build and is nearly ready for the trash.
Over a year ago I discovered "Soft Touch Polishing Pads" and ordered a set from Micro Mark and gave up on the sanding film which I had picked up at a local hobby shop. The pads come in a 6 pack, with 2400, 3200, 4000, 6000, and 12000 grits. They are 2" x 2" soft foam pads with rounded corners with the Micro-Mesh abrasive laminated to both sides and run $7.50 for the six pads. Only this past week did I finally have to order a new set; figure I got $7.50 worth of use out of them in over a year of sanding/polishing plastic. I swear by them; by the time I get down to the 12000 grit, what I've sanded/polished on a model actually looks smoother than the surrounding area.
Micro-Mark also carries sanding film which comes in a 10 pack, 4 " x 5" sheets, 2 sheets of 5 different grits. About all they say as to the grits is that they range from 23 micron down to .5 micron (equivalent to 10,000 grit) and include silicon carbide, aluminum oxide, cerium oxide, and chromium oxide. The 10 pack runs $4.35.
I'm not trying to endrose Micro Mark, nor do I receive any reimbursement, but I've dealt with them for years and have received good service.
Hope this helps.
Dick McC