If you're in the Seattle area, you can get this (and other similar styles and sizes) by the pound at Alaska Copper and Brass, near the Spokane St. viaduct and S. 4th Street, or something like that. I think they still have a collection of butt-ends that an off-the-street consumer can brouse through.

How I do it, (by which I mean I 'invented' this. I wouldn't be surprised if a real boat builder does something completely different):

Establish from the worn areas on your stems the length you want to protect, then add a bit on either end. The clear trade-off is weight vs. protection, as the brass is quite a bit denser than wood/glass/epoxy (or w/g/e).

With a compass-like scribe, scratch a line on either side of the keel line so that when the w/g/e is removed to that depth, the thickness of the brass will return the line to the original. This assumes that the keel will not be WIDER than the brass you are using when set to this depth.

Remove the w/g/e material with rasps, chisels, SureForms, whatever, to the line, or just a schosh beyond. Small dips are okay, as you'll be using thickened epoxy as a bed anyway; humps will make a high spot on the brass which will look even worse when you remove the excess to make it fair.

Cut the brass to length, and bend it by hand and mallet to conform as well as you can so that it will lay into the platform without needing additional pressure. It should be wider than the keel at the joint all along the length. Drill appropriate sized holes starting 1" from each end and 4" spacing between. I use Philips drive #6 x 3/4" oval head brass wood screws. Counter sink the holes to accept the bevelled bottom of the screw head. Pre-drill and seal the holes in the boat keel. Where the screws will penetrate the hull completely, you can either trim each screw or later cover the ends with little pyramids of thickened epoxy, being sure not to create little equipment-biting fangs in doing so.

Rough up the boat side of the brass with coarse paper and install on a thin bed of wood flour-thickened epoxy, along with the screws. After the epoxy has set, file off the excess brass sticking out on either side of the keel line. File off any portion of the screw heads that stick out.

I have a rather poor photo at my website (link below). Click on "Stern Keel Protection" in the left-hand menu.