Try these guys. http://www.northrun.com/ I know they say they are not a direct retailer, but we buy stuff from them all the time. Great folks to work w/ and prices are much better than most. Saddlery Liquidators could probably hook you up to. Happy hunting!
Bartville Harness makes a nice thong (three choices on length) and much more reasonably priced than others. (717)529-6992 no webste as they are an Amish run business but they will either take orders by phone or you can request a catalog and order by mail.
I get kangaroo thongs from a guy named Ad Beekman in the Netherlands. He is very nice and does nice quality work. Last one I bought was about $105 including shipping. Depends on the exchange rate. He has a choice of lengths and white or natural. I paid him with Paypal. He communicates well and ships quickly. Best deal for real kangaroo I've found.
once I opened the package I found the leather stiff and dry
a cup of needs-foot oil and rubbing alcohol 50/50 with the thong into a zip lock
flip occasionally for several days, drain, dry, install
smooth and pliable ever since