There isn't a lot of original source material for Welsh Border Morris. There's a book. If I hadn't already known it existed (I think the site where I originally spotted it must no longer have it), I'd never have found it.
The ROOTS OF WELSH BORDER MORRIS
⊕ ‘The Roots Of Welsh Border Morris: The Welsh Border Morris Dances of
Herefordshire, Worcestershire and Shropshire’, by the late Dave Jones,1988,
revised 1995. ISBN No. 0 9526285 0 3. £5 by post from: MRS. A. J. JONES
* Millfield, Golden Valley, Bishops Frome, Worcs WR6 5BN.
email anniejones at millfield dot orangehome dot co dot uk
I've just Paypal'd her the cash and she says she'll get it in the post to me.
Additionally, I conveniently bought a set of morris DVDs at Wimborne Folk Festival, and among them is a
DVD of Silurian Morris, who are the original Border revival side, of which Dave Jones (author of the book above) was a member, and dance all thirteen known collected Border dances in a style that's pretty close to how they were done historically. I'm only part way through it, but it's looking to be pretty good value for £7 (a massive £7.50 if you add postage). It's got some history, and complete dances, and a good feel for a side who were rather fond of shocking people now and then...
Their kit shows clearly the evolution of the typical modern Border Morris costume.

See the strips of coloured fabric pinned to their shirts?
Multiply that historical form of kit by a lot more material and you have a tatter jacket:

Next, go for proper colour co-ordination and you have more recent versions of the tatter jacket:

Now, to tart that up even further, here's what the ultra-modern Border side is wearing:

Interestingly enough, there are references to spangles on costumes over 400 years ago, so you can prove almost anything to be traditional in morris!
Of course, the dances have also undergone a similar evolution. The collected dances are relatively simple - they were often only danced around Christmas when the agricultural workers and fishermen needed the money - so not a lot of practised performance. Having only thirteen dances would be very limiting in any case, so there's been lots of innovation and some of the modern dances need a high degree of skill and are very exciting to watch. (the old ones are fun to watch too, but some of the new ones are fabulous!)