Ebay it:
New BMW E70 X5 Fender arch wheel Extension kit 20" 21":eBay Motors (item 130303305970 end time May-28-09 19:35:25 PDT)

As for install guide, I put it a summary out there (thx to others who've advised before me):
http://www.xoutpost.com/x5-e70-forum...er-flares.html

The plastic rivet tool can be found at the bottom of this thread for cheap:
http://www.xoutpost.com/x5-e70-forum...-retrofit.html

Shouldn't cost more than $250 total.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr