First,
Quote:
Yea ha, chicks and guns.
What about chicks and guns???

Next, do you have a recycling center near you? If you do, you have several options.
1. If it is an unmanned drop-off location find out from the owning entity if you may go through it. Tell them exactly why and just be upfront about it.
2. Post a sign at the recycling center of what you want. I think you would be suprised how many people will save them for you.
3. If it is a manned station tell them what you want and see if they will let you come in and sort.
As far as asking a government entity or private business for the envelopes, go for it! They will let you know if they have a privacy policy against it. Some of the places will even set them aside for you. Offer to sit at their office to remove the stamps you want so you are not taking the addresses with you.
Go armed with knowledge. Here in the US if the envelopes are normally thrown in the garbage without being shreaded, there is no reason why you can not have them. If you feel froggy, dumpster dive. Once a place or person puts their trash/recycling at the curb or in the dumpster (as long as you are not tresspassing to retrieve it) you can take all the garbage you want. The business or the person has given up their reasonable expectation of privacy once it is "abandoned" to be collected.