You don't really need a kit to build a crystal radio :-)
All you need is a BC tuning air-variable cap, maybe a trimmer, a germanium diode, a crystal headset or earphone, and some wire (actually, a lot of wire, unless you live very close to a transmitter)!
www.tubesandmore.com has everything you need.
There are also plans and designs for more and less sophisticated crystal radios (even FM!) on the web and in a variety of books (see the link above).
Maybe building a superregenerative receiver would be a little more interesting and give you a little more reception capability... :-)
EDIT: oops, except for the last sentence, you already got all of the above from earlier posts! Sorry :-P