I need a wiring diagram for a 1994 Chevy G20. Where can I get one for free or cheap?

You can go to Autozone.com, and obtain a wiring diagram.
If they have the Repair Guide for the vehicle, they'll have the wiring diagram also.
[I checked, they have the wiring diagram for 1994 Chevrolet G20 3/4 ton Van's]

To view their Repair Guides you have to become a member. It's free to join, and you receive no spam mail.
I have suggested using AutoZone's Repair Guides on here, to those who need access to a free online repair guide. I joined to check it out before recommending, and have never received any spam mail, or have had any problems.

On the home page for AutoZone, go up to the right, and click on -Become a member.
You'll be directed to another page, where you are asked some personal information.
I gave my real email address of course, but that was it.
Write down the user name, and password you have chosen.
After you have filled it out go to the bottom, and click on Submit.
You are instantly a member, and do not have to wait for an email.

Go to the home page again, and this time click on Log In.
After you have logged in go to the upper Left, and click on - Manage My Vehicles
Now go down to the bottom right, and click on - Add a vehicle, in the orange rectangular button.

Click on the Year (1994), then you'll be directed to another page.
Click on the Model (Chevrolet)
Now scroll down, and click on - G20 3/4 ton Van
Click on the Engine size used.

You will now be on the main page.
To the left you will see the heading -
Select a Category

Scroll to the bottom, and click on - Vehicle Repair Guides

Go to the right, and click on - Chassis Electrical - in blue.
Under the white - Select a Chapter - heading to the left, scroll down, and click on - Wiring Diagrams

If you are not familiar with reading wiring diagrams, you may want to view the first two instructional diagrams,
How to read and interpret wiring, and Common wiring diagram symbols.

If your van has a gasoline engine, the wiring diagrams for the 1994 G20's start at the 27th diagram down.
(If I have counted correctly. It is the weekend)

To enlarge the diagram you can either click on Enlarge, or simply Left-click on the diagram itself.

Use the bars on the right side, and bottom to move the diagram around, so you view it fully.