Generally, the most common tartans are easiest to find and the cheapest.
There are more than 10,000 (correct me if I am wrong) tartans registered, all of which may not actually have been woven.
The best option you have is to contact a mill directly, and enquire about the particular weight and finish (ancient, modern, weathered) they offer, and a minimum length they will produce for a private customer.
Alternatively, you can go via a kiltmaker who will quote a price for a finished kilt, although they may have a preferred mill which may not be the cheapest option.