We're missing way too much context here. Public or home? My home or someone else's? How many other people are nearby? Which body parts are involved? If this is a poop situation, how much and how firm? Is there a bidet/hygiene sprayer?

Depending on context there are many potential solutions:

• Waddle to another stall/undersink cabinet and look for spare rolls.
• Sacrifice "lefty" until you can get to the sink.
• Sacrifice undies forever, tossing them in the trash.
• Use the tube as-is.
• Use the tube after wetting it.
• See if there are paper seat liners that could be pressed into service.
• Or, y'know, ask for help. I understand that in certain conditions, three squares to spare should do it.
• Pull up pants and move along. Make extensive and unfriendly eye contact, implicitly daring anyone to say anything.
• Move into the stall permanently. You're a toilet-human now.