do you have an AP discount at your hotel?
is that the reason you want to buy them before you get to the hotel?
if that's the case, you don't really have to have them at that point.
If they ask you for your pass, you can say you're going to buy it as soon as you check in and that you'll show it to them as soon as you buy it...
i've never been asked for it in all our visits to WDW...
this year we bought them at guest services at DTD....
you can also buy them at any of the theme parks...
but i found DTD to be the easiest for us this year....no line at all....