I do not know where you live but I suppose with a Land Rover, it does not matter. I live in Western Canada and I got tyres and 18" wheels from these guys in Wisconsin a few years back - all good. Junk yards with wrecked LR3's are a bit scarce so you take from where you can.
Diamond Auto Parts - Quality Auto Parts and Auto Repair - Fond du Lac, Wisconsin
In your case, for the air suspension problems, I would say you will be purchasing new stuff as whatever will be wrong will be either the air spring assembly or some little thing. New rear air springs are to my knowledge only available thru Land Rover or independents like BPUtah. Arnott now has new front air spring assemblies but has not yet developed anything for the rear.
The rear air stuff is really crammed in so make certain you have frame jacks and the like supporting the rear. Assume that the air system is going to let go so make certain when that happens that no dropping would occur. This means both rear wheels off the ground, and the front end raised somewhat as well and supported on axle jacks so there is no lurching.
There are threads in the various forums on replacing the front wheel bearing assembles but think that you have to buy a new assembly rather than just pressing in a replacement bearing - others would know better, and you do not buy them used.
Re the headlight assembly, there should be is a sticker with the part number visible when you open the hood. Use that number when looking for parts plus find out the serial number from the donor vehicle - ideally it should be close to your vehicle and then it will most likely be similar.