Economical small septic tank for toilet and sink in pole building

[TurboMan]

Would like to add a toilet and sink in our new Equine barn which is about 300ft away from our house and about 250ft from our septic tank, Wondering if such thing exists. Or if there is another solution to this problem. Besides a "porta potty". After the septic tank It would not be difficult to tap into the leach field.