Download http://download.microsoft.com/download/vb50pro/utility/1/win98/EN-US/Msvbvm50.exe
Note that it's an http link so you'll get a security warning.
Double clicking on Msvbm50.exe will install the VB runtime which includes msvbm50.dll
Alternatively you can use 7-Zip to open Msvbm50.exe as an archive and extract msvbm50.dll