|Contrary to common belief there is no monopoly on what people can use as a bill of exchange. Monopolies make economies weak and subject to the whims of a few suppliers. You can use anything as a bill of exchange as long as it is agreed to by both the supplier and the customer.
The simplest type of BOE is a verbal agreement to supply something in exchange for something else. Today those agreements can easly be recorded on a cell phone. Just record the transaction using the phones video or audio recording feature.
It's important to remember that the value of the exchange is not determined by the value of the goods but by the needs of the parties.
The simplest unit of exchange is your time: You are given about 40000 seconds every day to exchange for goods and services.
Unless you convert your time to produce or seed there is not sure way to store or carry you time from one day to the next.
For an example of the wording that should go into a BOE see the "Public Notice" menu.