Hi Pam,
You could use a Teflon sheet or parchment paper even Kraft paper. The ink won't stick to the Teflon sheet, nothing sticks to them. If some ink does transfer to the sheet it can be wiped off and used indefinitely.
The thing you have to do is get the ink to 320 degrees Fahrenheit, that is when the PVC resin particles absorb the plasticizer and form a continuous film called an elastomer or fusion. If you do not reach that temperature the ink will crack and flake off the shirt when washed.
Bill M
