Need a zero-volume bottle? Searching for a one-sided surface? Want the ultimate in non-orientability? Get an ACME KLEIN BOTTLE!

From the "Important Information for Idiots" section:

Although originating in the 4th dimension, Acme Klein Bottles are immersed (not embedded) in 3 dimensions, using special techniques known to students of advanced topology. We strongly recommend that you do not attempt to evade the Pauli Exclusion Principle.

They (he) also sell(s) Klein Bottle hats.