Anything black will work. It is however so much easier to do both sides at once. I went to home depot and bought 2 black light tubes and a single ballast that runs 2 tubes. Made a 2 quick wooden boxes with a glass pane on one face. I lined the boxes with tinfoil to reflect as much light as possible. Then I used connectors directly to the tube pins to wire the tubes and ballast together. The 2 boxes have a couple of feet of wire between them so I can move the top box around. Then to do both sides at once. I print off both images and hold them together up to light so I can see through them. Then staple the 2 sheets together on 3 sides. Then its just a mater of slipping the board between the images, dropping them on the bottom box and placing the top box on top. Flicking on the power and watching the clock. Always comes out perfect with perfect alignment. You could use 2 similar sized cardboard boxes instead of wooden boxes to make things simpler.
@ panes of glass and 2 tubes and 1 ballast cost about $50