To the OP - 2 options for your cloth:
1) Before you start the instance, tell the group you are trying to level tailoring, and you would appreciate any extra cloth people have at the end of the run. They may need it for First Aid, so you may or may not be lucky.
2) Wait until the end of the instance, and ask people for any extra cloth they have. It will already be stacked up in their bags, so it makes for a quick trade (as opposed to trading Qty = 3 cloth after every encounter).
Other than that, you can always farm. Lvl 50 - are you looking for runecloth? Go to WPL and AoE the undead. You can get tons of runecloth there.
To address the slowing of a run by drinking...I think that was answered by everyone else.