Do you have access to a seller of plaid wool, just get 4-6 yards (most of my BPs are closer to 4 yards than 5 and I have a 38 inch pants waist and a 43 inch butt). The tartan does not matter as much, because the historical belted plaids pre-dated the clan-associations of tartans by a bit. I have paid as little as $36 for a BP, using plaid wool from the fabric store. The KEY is getting wool that has a "smooth" sevage, NOT the "normal" frayed one.

If you want a specific tartan, things get more expensive very, very quickly, as you move into worsted land quickly.