Welcome to the very best Cub Forum on the internet and to our wonderful Cub Family
Case Tractor Parts
or your local CaseIH Dealer will have that part for you. There are also some aftermarket ones offered by various vendors in RP, but ifn it were me I would stick with CaseIH. The quality is worth the little bit extra, and also, you won't be doing it again in a couple months.....
Oh, if you don't have the part number for the pump, the TC-37 series manuals are on the manual server at the link below. You will find them under the Cub Parts Manual link.