All of the samples I have sent were through Titan Labs. The only problem with them is the outrageous cost of the kits if you order them from their website. I've been able to find the kits at K-Marts and sometimes Wal-Marts for around $9.00, whereas they are $29.99 on their site. Go figure that one!!
If it comes to a point that I can't get them anymore I will use Amsoils. They are around $10.00 from any dealer and well worth it in my mind, especially if there are any concerns about your engine, trans. and rear-end.
The sample I just sent last month was mainly due to my concern of possibly having a bad head-gasket on my car. I recently have been having cooling problems on my 95 E320, and I wanted to rule out the head-gasket, of which the sample I sent in did. So as you can see, they can ease your mind a bit and help narrow the search down for a problem.
By the way, Arthur Dalton from this forum, helped me narrow the cooling problem down to indication. Come to find out I never had a real cooling problem, just a problem with my gauge!!