I cannot find anyone who sells cardboard airline garment travel boxes
to accomodate a garment bag- Some airlines have them at the airport,
but they don't want to give them to you if you are not traveling on
their airline.  Can you give me a source(s) where I can buy these?
There are many cardboard sellers, but I found none that sell this
specific product.

Hi bookshelf,

Are you in Florida by any chance?

Garment Box #4    30x15x4    $1.79
Garment Box #6    30x15x6    $1.89
Garment Box #9    30x15x9    $1.99
Garment Box #12   30x15x12   $2.09
Garment Box #15   30x15x15   $2.19
http://www.box-depot.com/garment_boxes_.htm
Box Depot, Inc.
2099 West Atlantic Blvd.
Pompano Beach, FL 33069
Phone  954-979-8326 Fax  954-979-8357
E-Mail BoxDepot@Bellsouth.net
http://www.box-depot.com/index.html

Delta Airline will sell you a box:
Garment Box  36" x 21" x 3"  $5
http://www.delta.com/travel/trav_serv/bag_info/sp_handling/boxes/index.jsp

Also, ask at your local moving & storage company - they might even
give you a box for free.

If you need the box for storing something valuable, such as a wedding
dress, please consider buying an acid-free archival quality box.

Medium Archival Garment Box 22" x 19" x 5-1/4" h $29.99:
http://www.containerstore.com/browse/Product.jhtml?CATID=175&PRODID=59868&FEEDCNT=3

Archival Underbed Garment Box: $37.99:
Large Archival Underbed Garment Box 40-1/2" x 19-1/2" x 6-1
http://www.containerstore.com/browse/Product.jhtml?CATID=175&PRODID=59861

Additional Links:

Kraft Garment Boxes 24x14x4 Case of 25 $18.33 
http://www.wrdisplay.ca/cclub/sept03/24xboxes.html

Lock Corner Garment Box White 24x14x4 Case of 50 $48.18:
http://www.wraptime.com/MM005.ASP?pageno=49

Care of Historic Costume and Textiles: Storage:
http://dept.kent.edu/museum/staff/care.html#Storage

Storing Wedding Gowns and Textile Heirlooms
http://dsc.discovery.com/fansites/homematters/crafts/990625craft.html

I hope this helps. If you have any questions or if this hasn't
satisfied your request, please post a clarification request before
closing/rating my answer.

Thank you,
hummer

Google Search Terms Used:

garment boxes
garment boxes archival
garment boxes travel

Outstanding- answer was prompt, accurate, and complete!  Thanks.

Dear bookshelf,

Thank you for your nice note, rating, and tip - all are appreciated.
I'm glad we were able to find the box you are looking for.

Sincerely, hummer