I got my brown glass bottles with child proof caps from the local chemists shop. They used to have 50, 100, 250 and 500 millilitre bottles behind the counter for dispensing things like cough medicine from larger bottles. Usually they would sell these for a few pence (although often demanded suspiciously to know what I wanted them for...)

Note I said 'got'... Past tense. Alas the trend now is to sell everything pre-packaged so many chemists don't mix up or dispense in the shop very much. Still worth asking, though, they may have hundreds of bottles 'out the back' they don't want any more ;-)

For 500ml and 1 & 2 litre sizes you need to ask for 'empties'. The cough syrup 'Calpol' is sold in 1 and 2 litre sizes, ask them to put the empties on one side for you. This is paracetamol in sugary syrup. Needs a good thorough wash, but nothing nasty or difficult to clean.

Last few bottles I got, just a few days ago, were from the local vet. Vets still dispense from bulk containers, so if the local pharmacist has stopped using bottles, try there...