Following my posts yesterday here are my thoughts on sourcing acrylic sheet.
One obvious source is businesses offering picture framing services: there are at least one or two in the Seattle area and I am attaching a link below just as a start. Picture framers, or whatever they term themselves, can be expected to offer both standard and anti-reflective acrylic, and should be able to supply pieces cut to size, however the one I looked at seemed fairly expensive and, whether due to Covid-19 or general policy, do not currently allow collection, so add $19 for shipping. As there are no doubt similar businesses all around the U.S. there would currently be no obvious advantage in using a local business.
Another possible source as mentioned above is eBay, in general an excellent source for many parts that can't easily be sourced locally. However, from my quick search most listings are for fixed sizes which may not be easily usable. And as mentioned, it is best to be alert for listings from businesses in China for the practical reasons that shipping may be very slow, and there may well be unknown extra charges for customs and shipper handling.
The next possible source, in principle, would be businesses that are plastic stockists and typically supply plastics for industrial uses, however those businesses are likely to be quite large and to have a minimum order value, although there is no harm asking especially if the business is local. There might however be a small chance they could suggest one of their smaller local customers who is buying the acrylic you need.
A further possible source in principle would be a business that buys suitable acrylic sheet for its own probably manufacturing business, but they are likely to be hard to find, although if you chance on one which is a small business they might be helpful, maybe even have suitable off-cuts.
But the most promising source I think would be sign makers, they typically supply signs to small business and are likely to be everywhere, maybe even one or two on the Olympia Peninsular, if there would be enough customers there. Being generally small businesses themselves and used to small orders, they should be easy to approach and deal with, and might well be willing to supply pieces cut to size at a good price. I'm not sure what thickness they wold normally stock but I imagine a reasonable range, however they quite possibly might not stock anti-reflective acrylic. If Yellow Pages is still published in paper form that should be a good place to start, otherwise an online search would be necessary. You might first check how those businesses are typically named to aid any searches.
Some assorted links from a quick search:
SeattleOnCanvas.com - Home
SeattleOnCanvas.com - Acrylic sheets
Tapplastics.com
PortPlastic.com
Those are just a few links from a quick initial search last week, lots of scope for more searchs if needed.