My turbo shaft-seals are also leaking... . drinks about a quart of oil a week. I checked with lots of turbo-shops via the internet and with follow-up phone calls before I found one I liked. If you go with a new aftermarket turbo, plan on spending $1,500+. If you go with a rebuilt unit, there are several shops that will rebuild yours if you send it to them... . or they'll send you a remanufactured unit and you'll need to pay a core charge (usually around $200) which will be refunded after you send your old one back to them.
Here's a list of possibles:
NEW:
BD Power @
www.dieselperformance.com
Diesel Specialists @
www.shopdieselspecialists.com
Diesel Power Products @
www.dieselpowerproducts.com
Xtreme Diesel @
www.xtremediesel.com
Industrial Injection @
www.industrialinjection.com
Jegs Auto Parts @
www.jegs.com
REBUILT AND/OR NEW:
XSboost Turbochargers LLC in North Lawrence Ohio @
www.xsboostturbochargers.com talk to Dave @ 330-830-4075
Midwest Turbo Center in St. Louis MO @
www.midwestturbocenter.com
G-Pop Shop in Springdale Arkansas @
www.gpopshop.com talk to Jim or Gerry @ 479-751-7966
Pure Turbo's @
www.pureturbos.com
EBAY:
Central Turbo's has a web-presence on ebay and they sell factory-direct 'new' turbo's to fit every truck... in their original 'Cummins' (Holset) box.

There are other shops that sell their turbo's on ebay as well... but you know the old 'Caveat Emptor'... buyer-beware!
OTHER OPTIONS:
If your so inclined, some of the shops above, will sell you a rebuild-kit and you can do it yourself.
Anyway... that gives you a starting point.
Do you homework.
Review their websites... then give the one's your interested in a call and talk to a warm-body and get the straight-scoop on the product your interested in. I had one all picked-out based on a TDR referral, but they were hard to reach on the phone... . then they were curt, abrupt and less than helpful when I finally got through... so I figured they were too busy to take my money and began my search elsewhere.
Ok... next you're going to ask what I did.
I followed my own advice and after reviewing my options... . their 'representations' and the price-ranges... and I settled in on a rebuilt HE351CW (standard for my 04. 5 truck) with a billet shaft and up-rated turbine & compressor wheels (ie: 60 X 70) for a little more air-flow. The shop I chose??? XSboost in Ohio. The cost? $895 for the turbo plus shipping... and I also sent an additional $200 core charge which they'll refund back to me after I send back 'my' old turbo. I worked with Dave on the phone and was happy with his attentive customer-service. He shipped the turbo to me within a week and my local shop (Underground Diesel) will install it later this week along with a new BD exhaust manifold (mine had 3 cracks in it). I also ordered new silicone hoses (4) for my pre-cooler along with new spring-clamps.
I trust that will get you started.
Glenn in La$ Vega$... .....