Geezus, you can use anything. Just don't flush it. Put a trash bag next to your toilet. If your toilet is next to the shower, just hop in the shower.
If you're out for a long run, I use a sock. You can even launder a sock and reuse it. Cut up old t-shirts. Cut up an unused bed sheet. Lack of toilet paper isn't a crisis.
When I see anyone buying more than a single jumbo-pack of toilet paper, I know that person is stupid.
For what it's worth, the last time I went to Costco, they had gotten a shipment in and had pallets of it stacked near the front door. I did not buy any. One jumbo pack lasts me about 3 months. Maybe even longer. I looked in the basement, and I must have just bought one around mid-February, and it's still over half remaining.