You mentioned the home you're interested in is off-market. Does that mean it is FSBO and the seller isn't paying a commission to your agent? If so, and if you also aren't paying a commission, then your agents reluctance is certainly justified in not working for free.
If none of the above is true, then yes, find an agent who is more helpful.
Regarding pre-approval. You can use whomever you would like. An up-to-date pre-approval submitted with an offer does not lock you in to having to use that lender. Keep in mind that switching lenders after an accepted offer could delay the process so make sure to keep things moving along quickly especially if your contract terms dictate timelines for how soon you need to submit your documents to a lender, obtain loan approval, close, etc.