Generally recommended--get the rest of your gear first, the pack last. When shopping for a pack, pack up your gear in a box and take it to the store with you. If you can, make an advance appointment at a slack time and tell them you want their most experienced pack fitter. That's because the pack must fit your gear and be comfortable for you with your gear packed inside. There is considerable difference between real live backpacking gear and a bunch of weights or sandbags.

As soon as you get the pack, load it up with your gear, a liter or two of water and the equivalent in weight/bulk of a week's food, and hike around the house with it (don't remove the tags!) for a couple of hours. Boring, yes, but you need to determine if the pack is a keeper while you can still return it.

More on pack selection in the articles on the home page of this site, left hand column.

I would get fitted at a locally owned hiking/mountaineering store if you have one. My experience with REI clerks (most of whom appear to have little knowledge of backpacking) has not been good.

My personal experience has been that the best packs are made by the various "cottage" industries who sell on the internet. Examples of lightweight framed pack (what I prefer) makers include ULA-Equipment, Elemental Horizons, Gossamer Gear, Six Moon Designs, Hyperlite Mountain Gear, among others. That's what I ended up getting. I was prepared to pay return shipping charges on half a dozen packs, but it turned out that the very first pack I ordered fit me as though it had been custom made for me! I did go through the store bit (hoping to learn what a well-fitted pack feels like) but found nothing there that I wanted.