I think I need to save a template of "bottle corking responses" so I can just paste it in every time this pops up.
a) as mentioned above the twisty offs are NOT designed for corks and can be prone to breaking. That being said I've never broken one and always end up using a few during each batch. I'm currently phasing them out though just to be safe. I have never used a different sized cork for them.
b) for gazillions of free bottles I recommend going to carabas or some other "fancy" Italian restaurant where you know they're serving wine. Ask to go through their recycling bin on Sunday morning. You'll have enough for several 5 gallon batches. bring gloves though... there will be broken glass and food nasties all over.
c) I always hate cracking open a bottle of wine if I don't plan to drink it all. And sometimes I'm just not in the mood for that much wine. So I do about half of each batch in beer bottles with crown caps for little single-serving brews. This also allows me to send out sampler 6packs with my friends rather then just one or two bottles. Not to mention that as far as cost goes caps are WAY cheaper each. You can even cap some types of sparking later bottles and many types of champagne bottles.