Here's one: Hold a 'beer-a-thon'.
Email 20 friends and invite them over for a party.
For the beer drinkers, apply one condition - they must bring at least 6 empty longies with them as a 'buy-in'. In exchange, they can drink your beer.
You'll get the empties and they enjoy your brew. I got 60 that way (I gave them 4 weeks notice. Easy to drink 6 longies at home in 4 weeks).
Here's another: Type up an ad and stick it up on the noticeboard at your local supermarket. Simply offer to pay 40c-50c (or whatever you're prepared to pay) for any empty longies. Leave you phone number.
You can throw in the line "your kids may want some pocket money" which gets them calling. You'd probably want to say on that ad that they should call first and talk about how many they can collect for you on average per month (most people know how much they drink as it tends to be a routine). Once you've got enough projected 'supply', take the ad down. Just make sure you call them as soon as you've reached your limit.
Got about 150 that way.
Good luck.