Most will work for a bonding company, and many bonding companies are backed by various insurance companies.
some privae individuals are allowed but will have to have enough resourse to back the bonds they are writing.
And no it is not the sheriff office but the court house, since the sheriff only takes the bond for the court, but it is the court that has the rules on acceptance of bonds.
If you have never been one, I would advice working for a company, to get experience on dealing and setting up rules as to who to bond and who not to.
If you are talking about being a bail enforcement agent ( they always work for a bail bondsmen)