Hey Richard - Wilsonart and Formica are the two brands you're likely to encounter.

It's usually sold in rolls. You can get it from Home Depot or Lowes. The rolls are quite large and intended for countertops. I went through my supplier to get a 10ft x 10ft or thereabouts when I did my office. It gets applied to a substrate, usually particle board or MDF, with contact adhesive and a roller.

Edit: Brains working now. You can custom order HPL as well from Wilsonart and Formica. When I was doing my research both options were available. But it will likely come in a big sheet like plywood does. What I did, and you might consider doing, is contact all the local distributors to cabinet shops and see who will sell to the public. I ended up having to create a fake business name to get an account. The distributors can get you any material you can think of and bonus is they deliver right to my garage next day for a nominal fee, like $20.

Matt

It appears these panels are used to fabricate washroom/toilet stalls and exterior cladding as well.  etc. I think that if you could find some fabricators that would sell you drops.

Most the companies that make these panels have regional sales reps that might be able to point you in the right direction and suggest some fabricators in your area.

Ron

That is most likely Phenolic, if you do a quick Google search you can many places you can small peices of it. It's great for templates but is nasty stuff to cut. Hope this helps.

It looks like the typical Phenolic panels that should be fairly easy to source if you have a decent sized cabinet shop in your city. Most of them should have some off-cuts, as long as you aren't too concerned about color.
The stuff is a pain to work with though. It is very hard and dense. Your regular carbide tools will cut it, but it will be slower. Don't even think about trying to drive a screw, nail, or staple into it. You can drill/countersink an oversized hole and screw through it, but not into it. It is actually dense enough that you can drill and tap threads into it, if necessary.
You can sand the edges to a very nice finish, if that is important, but it eats sandpaper pretty quickly too. If you need to attach pieces to each other, it really takes some physical joinery along with some epoxy type glue. Simple but joints don't hold so well.

Amazon has your 1/2” phenolic.  I’ve ordered if from there before, works fine.  https://www.amazon.com/Leecraft-BK-1-Blank-Phenolic-Sheet/dp/B000289556/ref=asc_df_B000289556/?tag=hyprod-20&linkCode=df0&hvadid=312137763108&hvpos=&hvnetw=g&hvrand=5236727440493434755&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9021117&hvtargid=pla-570657643098&psc=1&tag=&ref=&adgrpid=62149253716&hvpone=&hvptwo=&hvadid=312137763108&hvpos=&hvnetw=g&hvrand=5236727440493434755&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9021117&hvtargid=pla-570657643098

Gotcha-

Well, I've ordered full sheets of phenolic from these guys, they've got 1/2" 24"x48."  Pricey.

https://www.usplastic.com/catalog/item.aspx?itemid=34408&catid=679#anchorTechSpecs

These guys will cut to order:

https://www.professionalplastics.com/PHENOLICPAPERX

Looks like a 1/2"x12x36 will run you about 170.00 before shipping.

I used to use this stuff for playground equipment and have ordered it successfully from the companies above, but it has been a few years.