You can check prices on Compare Pharmacy Prices
They list prices across several online pharmacies.
I would also recommend having a look at MedExpress.
Where are you headed? We travelled through Kuala Lumpur (en route to the Borneo jungle) and accidentally forgot our pre-ordered malerone tablets at home. When we arrived in KL, we found you could buy them over the counter from a pharmacy and they were a quarter of the price in the UK.