New parts availability is pretty good on the Murray lawn tractors, even the older ones. Parts Tree and other websites list lots of stuff. Wal Mart stores carry most of the basics like belts and tires.
That Sentinel would probably be a farm/hardware store branded Murray product. There's only about a gazillion older Murray tractors lying around discarded to rob parts from. You could get your trailer out when there's a citywide cleanup and grab a couple off the curb.