I've made a targeting vector with three LoxP sites. It is very
convenient to prepare conditional constructs.I would like to kmow how
can I shell it to any molecular biology company (Clontech, Stratagene,
Invitrogen, Promega,...). I mean, there is smoe specific department in
the companies to contact with about this subjet?

I assume you want to "sell" it rather than "shell" it to other
companies.  Almost all major biotech companies have a web presence and
a button on their site that say "Contact Us", or something to that
effect.  I've included below some of the detailed information for
Clontech, and contact links for the other companies you mentioned. 
Once you're into the contact information, identify the "technical
support" folks, and let them know (by phone, email, or on-line forms)
about your innovation.

Good luck!

www.clontech.com/clontech/Contact.shtml
Technical Support 
TEL*:
 877-232-8995 
650-424-8222 X5
 
FAX:
 650-424-1064
 
EMAIL:
 Biological kits & reagents: tech@clontech.com 
Software: softwaretech@clontech.com
 
FORM:
 Submit your questions to Technical Support

****************
 
http://www.stratagene.com/contact.htm

http://www.invitrogen.com/content.cfm?pageid=2784&nv=1

http://www.promega.com/worldwide/northamerica.htm

Molecular biology isn't my field so I'm not exactly familiar with
intellectual property for it, but I would suspect that you'll need to
make sure you have done your homework on patent protection before
starting to talk to potential buyers. If you disclose anything that is
not yet protected (or publically disclosed), be sure you do it under
the terms of suitable non-disclosure agreement. You don't want to
accidentally give away what you're trying to sell.

I assume that you did this work within a scientific institution.  You
need to be sure what your institution's policy is regarding
intellectual property and technology transfer.  Also, if your work was
funded by an outside body, there might be constraints in that respect
as well. Did you do the work all by yourself, or are there colleagues
who may feel that have a claim as well?  In terms of patenting your
product, you might be better off pursuing this through your
institution which should have people familiar with the legalities of
the process.

With respect to contacting companies, it might be worth identifying
the person who is responsible to the Board for the company's R+D and
making an approach to that individual.