Dish will not sell HD receivers, you need to get them from a place like the DishStore or from a local retailer. Also, most of the HD channels are one 129 and mirrored on 61.5 so you will also need either your existing dish replaced or a wing dish put up.
Dish has ALWAYS been happy to offer you the option of buying any additional equipment beyond the maximum lease offer. They love folks who have to buy. If a sub wants more than the what the lease pays for, CSR's have always quoted a price for any additionals and are ready to take your credit card number to pay for it. In some situations, an extra box/connection beyond the leased can be as low as $90, a few years ago, anyway.