On my '89 420 SEL, a seal popped and I ordered the seal kit. When I got it, I called Auto Sports Unlimited in Zeeland, MI, and asked for a clean, dry box. Cost $60 plus shipping. http://www.AutoSportsUnlimited.com/
I recommend them only because I've actually been there and seen the back of the counter. Not too many places have their own rebuilding setup. I was impressed with the transmission case oven. Thery sell other stuff rebuilt, also.