Is it permissible to accept a loan for Hajj from someone whose source of wealth is questionable?

Hajj is not obligatory unless one has the means (istita'ah), and one is not required to borrow money to perform it. However, if someone borrows money and performs Hajj, their Hajj is valid.

Your ignorance of the lender's financial situation does not prevent you from borrowing from them, for the default assumption is that their wealth is permissible unless proven otherwise. Even if their wealth contains some doubt, it is permissible to borrow from them.