Unless you want wood, I would suggest one of the PVC products like Azek. Azek is available in 3/4" and 1" thick sheets (like plywood) that would eliminate your width problem. You don't have to worry about rotting and it take paint well. I don't think Home Depot (or the other big box stores) carry the sheets but maybe they can order it for you or you can call Azek and get the name of a local distributor.

If you want wood, you're not going to find a lot of choices in something that wide, especially at the big box stores. You could try a hardwood specialty store or maybe you could hire someone to glue up panels for you using a more appropriate species of wood and waterproof glue.

You referring to gingerbread type material?

If so, using a weather resistant wood like white oak or cypress would solve longevity problems. Both take paint well and would not rot quickly if the paint were flaking.

Would have to get those woods at a real lumberyard or specialty wood store. Their stock will come in 3/4" thick as well.

If the brackets are under roof, weather and moisture shouldn't really be an issue if they are fully primed before install. I'd probably use poplar or spanish cedar which are available in widths in excess of 16"
Casey

this place have the brackets you want

http://www.vintagewoodworks.com/brackets.html

Thanks. Should have mentioned what I posted in the woodworking forum: I can't order premade, because they have to be exactly the same, per landmark rules. I am experimenting with some scraps of Azek, which I understand is basically the same as Versatex, but the melting while cutting is a bit of a problem. Also the blizzard that results from routing, but I can just keep the vac running...
The brackets are 17.5" x 28".

Here is a link that might be useful:

From that pic, you really only need boards about 12" wide. Angle the part so the grain runs from the end of one leg towards the end of the other leg, not parallel to the long leg.

Personally I would just use MDO. Proper prep,installation and painting will see many yrs of service.

In the end, I did use Azek. It was easier to come by than any appropriate wood. I endured the blizzard when routing, vacuuming up after each piece. They came out great. I sanded all the edges to slightly rounded over, which gave them a more wood-like appearance when painted. I used SW Duration, and it did take three coats to really cover them. They are in place now, including where two were missing before, and I am very pleased with the result.

Next up are some spindles on a railing on an upstairs porch, which are much more intricate. I think I will go with MDO on them, as they are more protected, and would be harder to rout on the Azek.