At the airport. Costs you nothing. They are quite small - you can't have more than 200ml (I think) of any one type of liquid - so best to use the ones they issue as they comply with the regulations. They give them out to you as you go through to security.
advice from Manchester Airport's website (although I expect they're all pretty similar, would check on your departure airport website to be sure)
Liquids Passengers will be allowed to take liquids, gels and pastes within a container of 100mls or less in their hand baggage. These items must be contained within a small, re-sealable plastic wallet, which will have to be removed from their hand baggage and x-rayed separately.
The new rules stipulate that the plastic wallet must not exceed one litre capacity per passenger - thats around half the size of a standard A4 piece of paper and it must be sealable. The wallet can contain two or more items of less than 100mls/100g each but must not be cluttered to allow security officers to carry out thorough checks.