I just replaced the weatherstripping around my 2011 Expy 2 weeks ago. I primarily use the parts sites from two dealers to determine the parts I need: Haag Ford out of Ohio and Lakeland Ford in Florida. I've found that Haag offers better prices on most products while Lakeland generally has better parts diagrams. That said, I believe Haag Ford recently upgraded their site as I was researching suspension parts the other day and found their site much better than what was available from Lakeland.
Something to note when looking for weatherstripping (and other parts for that matter) is there are often multiple diagrams showing the same part. You'll want to look at each diagram to determine if any additional parts might be needed for the job.
Here are links to both dealers showing the exact part you're asking about:
parts.haagfordsales.com
2011 Ford Expedition 5.4L Triton V8 FLEX A/T 4WD EL Limited Sport Utility FRONT DOOR. GLASS & HARDWARE. Ford. SSV. w/ XL. w/o XL. XLT.
parts.lakelandford.com
Here's a link to a 3rd party company that sells OEM parts:
Shop in the parts listing - 2011 Ford Expedition Front Door Glass & Window Controls. Discounted prices and Hassle-Free Return on genuine Ford parts.
www.fordpartsgiant.com
All links were bookmarked for my 2011. You'll want to verify their the same as your 2008 (most likely they are).
As to the job, it's not easy. You'll have to take the door cards off. Then, the interior weather shield. You'll then remove the mirror assembly. You'll have to lower the window, remove a few bolts, remove the window, and then the weatherstrip. I added a few scratches to the window tint on the first window. The most difficult part for me was getting the window lined up correctly again so it would go all the way up. It took me a little while of adjusting things to get it to work again as there's a safety mechanism that will automatically drop the window by 25% or so if it thinks there's a blockage. I got mine working again, but I'm not certain it's 100% straight as I still hear a little whistle in the upper corner of the window while driving.
Good luck with your efforts! There are quite a few pieces of weatherstripping around our trucks. I've tried to replace all of them as there's soooo much wind noise when driving these days. It's driving me nuts.