One of the best places to look for old recordings is www.gemm.com,
which claims to list "19 million CDs, LPs, 45s and more from thousands
of sellers around the world!" It is comparable to www.alibris.com for
books.  Both sites list items held by many dealers, with ranges of prices
and indications of condition.  Someone recently was wondering about the
value of an old recording; I looked for it on Gemm and did not find it,
which shows just how scarce it is.  Gemm permits search by title, "artist"
(including composer), or label; there is an advanced search feature which
lets you get specific.

Amazon.com also connects up with out of print dealers for recordings and
will do a search for you within specified price limits.

My luck has been mixed on orders through both Gemm and Amazon.  What I
have found is that some dealers claim to have items they do not in fact
still (?) have in stock.  This can slow things down.  In fact I am still
waiting to see an item from an Amazon affiliate that first claimed to
have shipped it six weeks ago but which in response to my inquiry admitted
it was "backordered." Not cricket.  But I have mostly received in good
condition--and promptly--most items I have ordered this way.  Gemm rates
dealers with a star system right at the point of listing each item.
Amazon asks for a rating afterwards.

Jim Tobin