Gunsmiths and military armorers use Lacquer-Stik to fill lettering and sight markings. This is paint in a stick form.
To use, scrape the end with a knife to expose fresh material and rub into the stamps until they're full.
Use a paper towel with a dab of lacquer thinner to remove the excess from the surface, leaving the stamps full. Allow to dry an hour then apply a lube to prevent rust.
Another option is white appliance chip covering lacquer. This is sold in small bottles at hardware and appliance stores. Apply with a small artist's brush.
Finally, buy small bottles of model paints and use a fine brush to apply it into the stamps.