You do a search. You find people posting about them. Then you find a DIY here or on bimmerfest...then you find there are websites where dealers sell online parts at far less than your locak dealer...you go to their website...you enter your year, model. etc...it tells you the part number. You search Google with that part number to see if it is cheaper...you decide if you want OE or aftermarket/OEM. You order them.

If your mechanic said lowers are bad, why not replace that first? Uppers should last 100k+ IMO