I couldn't find a US grant, unfortunately. Luckily there are an awful lot of cheap laptops out there these days.
ETA: It's a good idea to look into discounts at vendors for sure, per DeeDee's excellent advice. Most or all of the big computer makers who sell direct offer educational discounts. I believe Apple computers may be a bit high-priced for someone on a budget (though there are many nice aspects of them too, including good usability) so I'd shop around unless you prefer Apple or money is not much of an object. Some others to look at might include Lenovo, Dell, etc. At each vendor's website check for outlet models. I'd also periodically check the fatwallet.org "Hot Deals" forum.
