How do I get a piece of plastic for photocopying without buying a package of acetate?

Ask someone that has it.

Ask around. If you or anyone you know works in an office, they may be able to graciously give you the one sheet you need.

Also, I’m sure Fedex Kinko’s will sell you one.

Any photocopy shop will probably be glad to sell you one or two, and they are packaged in smaller amounts at Staples and other office supply shops.

I checked with both Kinko’s and Staples. Kinko’s can copy onto ¼ inch plastic, which is much too thick for what I want. The smallest package that Staples sells is 25 sheets for $39.

@LostInParadise It sounds like you need a special size. At my Staples, they have an inhouse copy area where you could ask for a single sheet, but if the standard size won’t work for you, I don’t know what to suggest.

You might try a stationery or craft store.