Tamiya makes a set of modern armor decals. Runs about $5 for the set. Luckily, modern US armor markings are very easy to make yourself. If you have access to a laser printer, just buy a piece of clear decal film and and use a program like PowerPoint or MS Word. Pick the font that your computer has that looks "right" (print out on a piece of regular paper first) and make your own.
Also you can pick up rub on dry transfer letters and numbers at the local arts & crafts store, again picking the right font. Either apply directly to the tank or apply to clear decal film and cut out the decals an apply as normally.