There are quiet a number of reputable sources available to meet one's needs, provided a little effort is made in "searching" to achieve that desired goal.
Naturally this being a matter of knowing where to procure such items & to what ones preference in type and application (use) as they do vary considerably . Let alone whether one wishes to replicate a bygone era,namely "collectable" & or "vintage" through to the latter day versions,however generally all are equally effective, dependant upon choice.
I trust the list set out below assists in some way :
1 ) AGRICULTURAL EQUIPMENT DISTRIBUTORS -- Specifically dedicated livestock.
2 ) EQUINE SUPPLIERS -- Mainly catering to the horse breeding / racing industry. The larger thermometers being available here.
3 ) SCIENTIFIC (Laboratory) INSTRUMENT SUPPLIERS -- Retailers / Wholesalers .. Here you may find thermometers that are designed to meet many applications, obviously manufactured to meet exacting standards in Long ,Short,Broad & Narrow...Well worth ones time investigating.
4 ) VETERINARY CLINICS & RESPECTIVE SUPPLIERS (to same.) -- Equine suppliers (see above) being the more direct route.
5 ) EBAY as a number have suggested !! -- Although again it is simply a matter of knowing where to locate / access said goods as there are several areas not necessarily recognized (& visited ) by some, so hence they can remain somewhat obscure in "search" terms. The best I have found are ..a ) Medical Instruments (used & new) using the dropdown to right of search panel & clicking onto "collectables", then when located l go to sub category (upper left hand side of web page) & click onto --- Science & Medicine 1930's till now .
b ) Surgical instruments (used & new) -- Simply use the same method as described in part 5a immediately above.
c ) Highlighted along the top bar of each page are additional search term recommendations -- There one may find many
more unique items along these lines to satisfy ones interest .
Significantly the rewards are there !!