Nice and easy:
1) e-bay is always a good place to find pre-releases.
2) Craigslist, believe it or not. You might be surprised what you can find in there.
3) People around the forums of every blackberry website (You never know who's really who, so pay close attention to people that make a couple of comments on the "upcoming" devices forums).
Always have this in mind: IF IT SOUNDS TOO GOOD TO BE TRUE, ITS A SCAM.
How to Avoid Getting Scammed:
1) Always ask the seller for references (Who have you sold phones to?, and do you mind me asking them about your transaction?).
2) Ask for a picture of the phone next to a piece of paper with your "Nickname" on it.
3) Ask for pictures of the back of the phone, the front of the phone, the sides, and the camera. (This way you won't get a broken/scratched phone).
How to Make Sure You Are Getting A Fully Working Phone:
1) Ask the seller to make a video of the phone texting you, using the video camera, the camera, and the ABOUT SCREEN.
2) Ask the seller to send you a PIN MESSAGE, then reply to that PIN MESSAGE from your current phone, and finally ask the seller to take a picture of the phone you want to buy showing your PIN MESSAGE on the screen.
* I AM NOT RESPONSIBLE FOR WHATEVER HAPPENS IN YOUR TRANSACTIONS, NOBODY IS ASKING YOU TO BUY THESE PHONES, THESE ARE JUST TIPS.
If you have any questions regarding these few points, please comment on the blog, and I'll be more than happy to help you.