Yes, they can apply at a Canadian Consulate in the USA.

Signature
Jim Humphries,
Former Visa Officer
> Hi,
> My parents are visiting me in US on a B-1 Visa from Pakistan. They
> hold a Pakistani Passport with a B-1 Visa. They also want to visit
> Canada. Can they apply for Canadian Visa from DC or Buffalo or they
> have to apply from their home country.
> Thanks,