If you can't find a new radiator, flush your existing one with a radiator flush kit (usually oxalic acid and a neutralizer). Check to make sure the clutch fans is operating. The clutch tends to quit after 5 years or so. Also check to make sure your radiator fins are clear of bugs, etc. Most automatic tranny coolers are built into the radiator, so check your transmission fluid level and make sure it's full. Low tranny fluid can cause the transmission to run hot which in turn sends overheated fluid into the integrated cooler/radiator and thus raise the water temperature. Check to make sure your water pump belt isn't slipping.
If you can, you might check to see if your engine is running with an incorrect fuel/air mixture (assuming you have a carb). I believe that running a lean mixture can cause the engine to run hotter.
Finally, since you have an '88 (and presumably a closed cooling system), check to make sure the plastic pressurized coolant bottle isn't leaking. Sometimes the caps on those things give up and slowly leak coolant. The fact that you changed the coolant also suggests the possibility that, at this point anyway, you may have air trapped in the system which needs to be burped out. The closed systems can be very finicky about not tolerating trapped air.
Hope this helps.
Erik
88 Comanche