My advice to you is this,
All delivery firms however small or large, be it royal mail or your local newspaper will have good and bad workers
the fact is its a hard very boring job with little reward so most people dont have the motivation to deliver them, however some people are more than happy doing it, so to go to any agency is a gamble in my opinion.
The best way to find someone is to get in your car or van and take a drive about, it wont be long before you spot someone delivering leaflets, now if they have got a face like a slapped ar$e and are dragging there feet down the road, there probably not the ideal candidate so keep looking.
In time you will spot someone doing it with a big smile on there face, whistiling to thereself as they work, walking down the paths not over the grass, shutting all the gates behind them. Now this is more promising
Now spend 10-15 minutes following them just to be sure that they dont miss houses, and then pull up and talk to them
pull out a big stack of your leaflets, and ask the guy or girl if they would be interested in making some extra money.
The fact is there doing it anyway and are allready happy doing it at whatever rate there being paid, so this money from you is a bonus.