I am in need of 6-type 500 Sangamd #500-3613-02, 9,000 MFD, 35 VDC
capacitor or the equivalent for my welding machine.  Where could I
find this part?

wally...

The only Sangamd capacitors I found were in auctions, so I
assume they've gone out of business.

I was able to locate an equivalent at a reasonable price on
the Galco Industrial Electronics website. As it turned out,
Galco was the only site returning any results at all in a
search for a 9000 MFD Capacitor:

MALLORY CAPACITOR, 9000 MFD, 40VDC, CAN ALUMINUM ELECTROLYTIC,
-10/+75%
Item Number: CGS902U040R4C-MALY
TOL COMPUTER GRADE, 1.375" DIA X 4.125"
Price: $ 16.36
http://www.galco.com/scripts/cgiip.exe/WService=Galco-Web/WA/wcat/itemdtl.r?listtype=Catalog&pnum=CGS902U040R4C-MALY

And, on another page which lists the same part, it says:

"*Prices quoted for minimum sell quantity, item price for
  additional quantities may be lower."

An email address for further assistance is also given:
mailto:salestech@galco.com
http://www.galco.com/scripts/cgiip.exe/WService=Galco-Web/WA/WCat/Catalog.htm?listtype=Catalog&search-part=CGS902U040R4C&search-name=MALLORY


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Searches done, via Google:

"9000 MFD" capacitor
://www.google.com/search?q=%229000+MFD%22+capacitor

Just a pointer in the right direction: Try Mouser Electronics at
http://www.mouser.com.  I've been leafing through their 1,300 page
catalog that's situated on my desk right now, but at the moment I
haven't been able to find the exact capacitor you need.  I've found
10,000 MFD, 35 VDC capacitators, but the 9,000 MFD/35 VDC seem to be
hard to come by.  I find their catalog a bit easier to use, but you
might want to head over to their site sometime and just check around.