I saved many 750ml bottles as long as 8 years ago. I like to plan ahead
Most home brew shops sell bottles for $1 each. While my batches are brewing I buy good beer in 750ml bottles by the carton. I think I've bought at least 3 cartons since I joined here, so that's 36 of them.
The old bottles collected over the years were about 120.
Plus I've bought some cheap PET bottles. Nothing is wrong with PET is you are not going to age them more than 6 months. HB shops sell them for less than $1ea and you don't have to worry about a capper. Coopers even gave them away in their HB kit here.
PET is good for beginners, don't have to worry about bottle bombs, can easy feel the bottle carb up, if you over carb or bottle to early you can easily release pressure and reseal. You don't have to worry about cappers, it's easy to see the yeast when pouring as it settles in the 'feet', no chance of chips and breakage.
I bottle in a mix off 8 year old twist top 750ml bottles, coopers crown seal 750ml bottles bought recently, 750ml brown tinted PET bottles and 500ml crown seal german lager bottles (collected them when I wasn't HBing too, a six pack at a time over a year).
All in all I have enough bottles for 6 x 20L batches.
edit: also check your local classifieds, lots of brewers collect and then give up or switch to kegs, so there must be private sellers out there with bottles to sell.