I don't understand your reference to an air lock, but they do need to be able to handle CO2 pressure produced by the bottle fermentation.
Your options are plastic or glass.
Any plastic bottles which have held fizzy drinks should be suitable, and you also re-use the caps. Typical examples would include 5/600ml cola bottles, 1 litre dry ginger / tonic water etc. mixer bottles and 2 litre pop bottles. 2 litre fizzy mineral water bottles would also be ok, the âvalueâ types are usually around 16p if you wanted to buy them just for the bottles and ditch the contents.
If you want to use glass bottles you will need to buy a crown capper and a bag of caps, but should be able to get the bottles for free if you ask at your local pubs or scavenge around recycling centres. Magner's or Bulmer's bottles are the best, they hold proper pints and their labels soak off easily in hot water.