Two big brands are Evergreen and Plastruct.  You can find it at most bricks-and-mortar hobby shops, including model railroad shops.

As far as online sources go, bypass Amazon and go right to the source.  You can buy direct from Evergreen via its website, https://evergreenscalemodels.com/

and from Plastruct via its website, https://plastruct.com/   Plastruct also makes liquid styrene cement, in two varieties, Weldene and Bondene.

There is another brand, more known to model railroaders, I think: K&S. I'm not sure if they sell retail, though.  I know that you can buy their product from online and bricks-and-mortar retaliers.

Also, MegaHobby carries a good supply of styrene sheet and shape stock:

[URL unfurl="true"]https://www.megahobby.com/categories/scratch-building/styrene-sheets-and-strips.html[/URL]

Finally, for some gauges of styrene sheet, try your local hardware store, and look at yard signs-"For Sale", "For Rent", "Keep Off the Grass", etc.  Many such signs are printed on styrene stock, and are cheaper by the sheet than the materials made for hobbyists.  If the surface will be exposed, you might have to sand the color off, but for interior work or general utility work, it's just as good as product from the hobby companies.

Hope that helps!