You can get a Customs bond through a surety (essentially a sponsor) licensed by the U.S. Treasury Department.
How to Get a Surety for your Customs Bond
You can get a Customs bond through a surety on your own. To do so, start with this list of sureties at the Treasury's Financial Management Service website.
You can also receive a bond or surety through many customs brokers. You can find a list of the brokers in your state here. Note that some brokers require you to give them power of attorney in order to buy bonds from them. Power of attorney allows the broker to file your entries on your behalf.