I would expect a darkslide slot leak would be some sort of wispy smear that cuts off short of the film edge, and maybe even angled depending on the external light conditions. (But I am working more on analysis than experience here! )
As to film, cheap is a relative term! The faster the film, the more sensitive the test, but it's probably silly to go for 1600 or more. I would probably opt for an ISO 400 like TriX, Tmax400, HP5, or Delta 400. If you have to send out (and are in the US) one of the Freestyle films -- maybe Arista EDU Ultra 400 -- that's pretty inexpensive.
Hope you find it soon; that can be pretty frustrating.