That depends on which toilet you are in!
Is it the WC or is it the bathroom, If the WC prior preparation and planning would have seen you stock the small cupboard with reserve supply! a short lean to the left or right should see you right!
If the Bathroom then you may have a far further reach to the under sink cupboard, if this is indeed the case there is no other option than to kick off the troosers and skiddies, clench the cheeks and try to step into the shower/bath without releasing said cheek clench, once there release clench turn on water and douse.
P.S. Replenish said toilet roll before departing area.