There are many sources for the selection numbers of the CD titles carried by the clubs. These include advertisements, monthly flyers, catalogs, and the customer service centers. The problem with the ads, flyers and catalogs is that they are not comprehensive. The problem with the customer service centers are, while they are the definitive sources, it is inconvenient to have to call them all the time.

Thankfully, there are online catalogs. BMG and CH/CDHQ/PLAY have official web sites with searchable catalogs including almost all club CDs. An unofficial site, www.cd-clubs.com, has searchable and downloadable catalogs for BMG and CH/CDHQ/PLAY.
The catalogs of the same named club from two different countries (e.g. BMG US and BMG Canada) are not the same. <insert club name here> Canada members can not order from or get selection numbers from <insert club name here> US catalog and vice versa.

BMG Music Service:

Official: WWW: http://www.bmgmusicservice.com

Unofficial: WWW: http://www.cd-clubs.com

CH Music Club / Play from Columbia House:

Official: WWW: http://www.columbiahouse.com

Unofficial: WWW: http://www.cd-clubs.com

BMG Classical Music Service:

Official: WWW: http://www.bmgmusicservice.com

Unofficial: WWW: http://www.cd-clubs.com

Unofficial: FTP: ftp://ftp.gmd.de/music/cd-catalogs/bmg-classical-collection.gz

BMG Sound & Spirit:

Official: WWW: http://www.bmgmusicservice.com

Unofficial: WWW: http://www.cd-clubs.com and
http://www.users.csbsju.edu/~eknuth/bmg-ccm/index.html

Selection numbers are generally good indefinitely. Titles do become unavailable for two main reasons: lack of demand or lapse of license to sell the title. If demand is too low, it might be uneconomical for the club to do another pressing of the title. If the license to sell lapses, and the club can not renew the license for the title with the artist/label, the club may not continue to sell the title.

Selection numbers from most any source (advertisement, monthly flyer, catalog, etc) can be used when ordering CDs from the clubs. BMG's 5, 6 and 7 digit selection numbers are interchangeable. To convert a 7 digit selection number to 5 digits, lop off the first and last number. To convert a 7 to a 6, lop off the last number. To convert a 6 to a 5, lop off the first number. Here's an example:

Eric Clapton's "Cream of Clapton"
Seven digit code (from current listing) : 1082411
Six digit code (from main print catalog) : 108241
Five digit code (from magazine advert) : 08241

+ CH's, CDHQ's and PLAY's catalogs and selection numbers are identical.

The artists and titles in the clubs are dependent on the contracts between the clubs and the record labels and on the contracts between the record labels and the artists. If a label has a contract with a club, most of that label's new artist contracts will include a clause allowing the label to license the artist's work to the club.