do an ebay search for 7" brake boosters, you can get a dual chrome 7" booster w/chrome master cylinder for about $160 shipped. I have one but have yet to put it on, i heard the master cylinder is shit, though.
I GOT A 7 INCH BOOSTER FROM DEVIOUS CUSTOMS. ITS ON BUT I STILL HAVENT DROVE THE TRUCK IT FITS GOOD THOUGH. YOU ONLY HAVE TO MAKE AN ADAPTER FOR THE ROD GOING TO THE PEDAL. THEIR WEBSITE IS WWW.DEVIOUSCUSTOMS.COM