Abstracted from "The Encyclopedia of Western Lawmen & Outlaws" by Jay Robert Nash:

Port Stockton---born 1864, killed January 10, 1881 near Farmington NM.

Ike Stockton---died shortly after September 27, 1881, possibly in or near Durango CO.

A check of their death records may provide information.

From the 1880 federal census for Animas Arriba, Rio Arriba Co., New Mexico:

Isaac STOCTON Self Married Male White age 29 born TEXAS occupation Stockman father born TENN mother born TENN
Alan STOCTON Wife Married Female White age 22 born TEXAS occupation Keeps House father born MISSOURI mother born TENN
Delilah STOCTON Dau Single Female White age 5 born TEXAS
Guy STOCTON Son Single Male White age 4 months born NM

This might be your Isaac Stockton. Nothing for a William Stockton.