Sources of out-of-print fiction include:
Joint Fiction Reserves (JFRs)
There are three JFRs in the UK and Ireland: the Regional JFR, the London and South Eastern Regional Joint Fiction Reserve (LASER JFR), and the Irish JFR.
These schemes all operate in a similar way; participating libraries aim to acquire all new fiction published within a particular alphabetical allocation and to make this available for interlibrary loan.
A similar but distinct Scottish Fiction Reserve is also in operation; the Scottish JFR allocates authors to authorities on the basis of local association
For more information, see the Guide to the Joint Fiction Reserves 2014
National libraries catalogues:
Online retailers such as ABE books
Online services such as Fantastic Fiction
Subscription libraries such as the London Library