The Least expensive in a Tartan Kilt is Stillwater, and also Frugal Corner. I have a tight budget (Vow of Poverty) so can go tossing away hundreds on an article of clothing, God would box my ears. Lol.
I have several of their kilts, plus Amerikilt & Utilikilt (they are not tartan) Kiltstore and Heritage of Scotland are a little more in price, but is best to buy on Ebay as it seems to avoid high shipping etc.

Welcome, by the way.