You can find a quite complete collection of Johnson's works in electronic form on this webpage. If reading aloud is your fancy, I quite recommend his essays from The Rambler.
Among his works, I stumbled upon this, a 1777 legal brief filed on behalf of a slave seeking freedom:
It must be agreed that in most ages many countries have had part of their inhabitants in a state of slavery ; yet it may be doubted whether slavery can ever be supposed the natural condition of man. It is impossible not to conceive that men in their original state were equal; and very difficult to imagine how one would be subjected to another but by violent compulsion. An individual may, indeed, forfeit his liberty by a crime ; but he cannot by that crime forfeit the liberty of his children. What is true of a criminal seems true likewise of a captive. A man may accept life from a conquering enemy on condition of perpetual servitude; but it is very doubtful whether he can entail that servitude on his descendants; for no man can stipulate without commission for another. The condition which he himself accepts, his son or grandson perhaps would have rejected. If we should admit, what perhaps may with more reason be denied, that there are certain relations between man and man which may make slavery necessary and just, yet it can never be proved that he who is now suing for his freedom ever stood in any of those relations. He is certainly subject by no law, but that of violence, to his present master; who pretends no claim to his obedience, but that he bought him from a merchant of slaves, whose right to sell him never was examined. It is said that according to the constitutions of Jamaica he was legally enslaved ; these constitutions are merely positive; and apparently injurious to the rights of mankind, because whoever is exposed to sale is condemned to slavery without appeal; by whatever fraud or violence he might have been originally brought into the merchant's power. In our own time Princes have been sold, by wretches to whose care they were entrusted, that they might have an European education; but when once they were brought to a market in the plantations, little would avail either their dignity or their wrongs. The laws of Jamaica afford a Negro no redress. His colour is considered as a sufficient testimony against him. It is to be lamented that moral right should ever give way to political convenience. But if temptations of interest are sometimes too strong for human virtue, let us at least retain a virtue where there is no temptation to quit it. In the present case there is apparent right on one side, and no convenience on the other. Inhabitants of this island can neither gain riches nor power by taking away the liberty of any part of the human species. The sum of the argument is this :—No man is by nature the property of another: The defendant is, therefore, by nature free: The rights of nature must be some way forfeited before they can be justly taken away : That the defendant has by any act forfeited the rights of nature we require to be proved; and if no proof of such forfeiture can be given, we doubt not but the justice of the court will declare him free.Johnson was a staunch Tory, conservative by temperament, and had little sympathy for the aspirations of the American colonists. In his tract Taxation No Tyranny he combined his dislike of radical politics with his dislike of slavery in commenting, "How is it that we hear the loudest yelps for liberty from the driver of Negroes?"