Replace the whole transmission or just the valve body gasket?

My dads friend told me just to get a transmission and replace the whole thing because it is easier but he thinks my cars problem is the valve body gasket and i have a 2005 ford mustang but i can't find any where to purchase a valve body gasket for this car. If you know drop links below. But the car has shifiting problems. What should i do. (its not the solenoid)

What's the problem with the car. Gaskets don't cause major problems with the transmission

If your Dad's friend told you to jump off a cliff…
Why not get a real transmission tech to have a look? It could be something a lot easier than doing the whole transmission.
I just came home from helping a carpenter friend put his Ford truck timing cover and water pump back on because after he changed his thermostat he thought the timing cover was leaking. He had the thermostat flange pinched in the housing. He blew his whole weekend chasing his tail and now owes me a boat load of beer ;-)