It depends on the situation. If the emotion you feel now is disappointment, then "I feel disappointed." is best. If you are talking about some event in the future or some event that is a possibility that makes you feel disappointed, then the correct form would be "I get disappointed when A" Examples: "I get disappointed when my plans don't work out." "I get disappointed when I get a bad grade on a test." You would not say just "I get disappointed." The question you would be asked would be "What makes you get disappointed." One more point. If you are in the process of becoming disappointed and the feeling is increasing, you could say "I'm getting disappointed." "I've had three interviews this week and no one has called back. I'm getting disappointed."

I Feel disappointed = you're already feeling disappointed. I get disappointed = you will be disappointed in the future. However, you can also say "I feel disappointed when I get yelled at." Both are actually okay for future tense. Or "I become disappointed"