I bought one from Harbor Freight for about $40 - works fine. That being said, I'm not sure that it's really necessary. I believe that if you warm the transmission up enough with a nice drive before you check it that your temp is going to be around 80°. I don't think it gets much hotter than that. Someone correct me if I'm wrong please.

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Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles