You can get a bill of sale notarized by a notary public. These are individuals authorized by the government to perform various legal functions, including verifying and authenticating signatures on documents. To find a notary, you can typically check at banks, law offices, government offices, and other official places.
It is important to make sure all the requirements for the respected state law are included in the bill of sale. As, with any legal written document a bill of sale can be handwritten.