Well I don't know where you can get clothing but if you are intrested in makeup M.A.C has a hello kittly line coming out on the 12th! Im pretty excited for it.
tons of stuff
at hot topic.
at the mall I go to, the also have a booth of hello kitty.
At the MALL!!!
I bye all my Hello Kitty stuff there!!!
I am obsessed with Hello Kitty! Omg:D
You can also find it at Japanese stores...and Sanrio Stores=)
Hot Topic has loads of stuff, as mentioned above^
Lot's of sales as well.
Good luck on your search for Hello Kitty merch!