Bookpoint sells all types of 2nd hand learning materials.
Bookpoint buys from students who have completed their studies
and no longer need the materials and then sells these materials to
current students eg Open university students who have completed
their studies might sell all their learning materials to Bookpoint
and Bookpoint will then sell the course materials to current Open
University students.
If you really cannot find the Open Course materials for your
optimisation module, you can refer to Alpha Chiang 2 books on
optimisation by McGraw Hill. These 2 books are used by NUS
undergraduate and graduate students for their optimisation
modules.