As far as I can make out, both Amazon.com and CDNow allow you to search
by catalogue number.  It is possible however that the catalogue numbers
they use do not match those of British or even US record companies and
distributors.  However, you can always search for a specific CD by
specifying composer, work, conductor, soloist, label, etc., to find the
CD in question.

With Sony, a digit between the M and the K in the SMK prefix always
indicates a multi-CD set, and specifies the number of CDs in it.

While we're at it, someone on another list recently recommended
www.everycd.com.  Naturally, I have no affiliation with them, but I do
notice you can search for items by catalogue number - though I've no idea
What's more, they seem to offer competitive prices and a good selection of
hard-to-find labels.  There is a $40 US annual membership fee, but this
can be cancelled at any time during a one-month trial membership period,
regardless of whether you've placed an order or not.

Best
Robert Gray
Kinograph, Montreal