Acrylic sheet comes in two forms: cast and extruded. Cast acrylic sheet has a relatively wide tolerance on thickness.. as much as +/-.010" (+/-0.25mm). Extruded sheet has better tolerances, but costs more.
There are many plastics distributors.. Tap, Regal, Interstate, etc.. and you can probably find at least one in any reasonably large city. Their bread and butter trade is advertising substrates, but they can order anything in their catalog from a central warehouse if they don't keep it in local inventory.
And when in doubt, McMaster-Carr is the Maker's friend:
They have a broad inventory and ship quickly.