Many years ago I reclaimed a lot of old flooring. I don't do so anymore. Even if you get the old flooring for free the cost for re-milling it and the labor cost to re-install it far outweighs any savings you might get from simply buying new and installing it instead.

Douglas Fir is probably the hardest to reclaim as it becomes extremely brittle over time. Long Leaf Heart Pine is one of my favorites for reclaiming since it looks a lot like Fir and has very similar strength characteristics. Douglas Fir doesn't have the high pitch content that Long Leaf Heart Pine has. This heavy pitch makes the pine quite at lot more durable with heavy foot traffic even though it is soft (similar to the Fir). It also helps it to reclaim better than Fir. Most of the reclaiming mills has strayed away from Douglas Fir as a reclaimed product in favor of Long Leaf Heart Pine due to these factors

Expect to lose at least 25% of the flooring you try to pull up even if you are extremely careful due to broken tongues, broken and cracked boards and other things too numerous to mention...but you'll see.

You'll find dirt, filler, finish and other debris will be stuck to the edges of the boards down their lengths that won't be removed even if you sand the floor before you remove it. Of course if you don't sand it first, you will also have a lot of old finish adhered to the faces of the boards as well. I don't recommend that you sand the floor before you pull it up. Settling, shifts and sags in old buildings can cause you to lose a lot of good sanding face from the old flooring if you sand it clean before removing it.

If you can however, I would suggest you re-mill the flooring before you re-install. Not doing so will cause you to have lots of ugly cracks caused by the variable board faces from all this old debris.
Some people try hand scraping or sanding this debris off the boards before they are re-installed. This can be a real pain. As you will not be able to hold a straight line. Plus it's a dirty thankless job that doesn't work very well.

If you decide to re-mill, you must be extremely careful making sure all old nails or other fasteners have been removed from all the old flooring first. Even so, the old finish and other debris will play havoc on yours' or someone elses' planer blades.

Finally, all the resources that I am aware of for salvaging or reclaimed flooring are folks who are selling flooring that's been re-milled from old lumber (oftimes old floor boards 1-1/2 to 3 inches or more thick) and other timbers taken from old buildings. Some are even cutting new lumber from old sunken logs taken from lakes and river bottoms. A few of these guys used to try to salvage flooring without re-milling it but found (as I've already explained above) that it's just too labor intensive and overall not cost effective to persue.