I say go for the nuturing parent. Be it mom or dad, Hop/hobble/limp to a room without anyone else being able to hear you, Then just buck up and ask them to buy you a pack so you can "give them a try".
explain the facts, leg hurts, IBS and all it just makes the right decision no matter what the embarassment level is.
if they say no, then no harm done, if they say ok, tell them what size you think you need.
If the straight on approach won't work, try the sneeky one, wait until you know someone will walk in the room and have the depends website on the monitor http://www.depend.com/products/individual/fitted.asp when asked about what your looking at, tell them you thought it might help keep things cleaner and neater