I agree with Todd. If you want to do this for "little or no" cost to you, your best bet is to borrow some garb from one of the members of the reenactment group.

By the way, I hear it said all the time that "just because the first documented evidence of the great kilt was 1594 doesn't mean it wasn't worn before that date." Which is perfectly true, of course. However, usually it is being said in order to justify wearing the great kilt for reenactments from 100 or 200 years prior to the date in question. While I would accept someone wearing the great kilt for a period of a decade or two before 1594, for the reason stated, there really is absolutely no reason to assume anything like the great kilt was worn in the fourteenth or fifteenth centuries.

What would have been worn by the Highland Gaelic Scots at the time would have been very much like contemporary Irish dress. What would have been worn by Lowland Scots would have been very much like contemporary English dress -- which you can easily find on the web site that I referred to.