Miata won't start some days. The garage checked the battery a d the alternator and they are fine. I jump started it. They suggested that the next time have it towed to them. Any ideas?

Something smells here. If it started, after you jumped the battery, the battery is likely the problem. I question how they tested the battery--amperage is what matters. I would remove the battery and place it on a battery charger, which most auto parts stores will do for free. Only then will you know its real condition. My bet is you need a new battery. Be sure to clean the battery and cable connections, while you're at it.

Once you're done the tasks TxGreaseMonkey, recommended, you'll probably want to test the car for excessive battery draw while the key is off.
Sometimes, simply a light staying lit in the glove box or trunk or such is enough to give you fits, even though the thing is shut tight.