pixel_player94711205 First you do not have to send ships to the museum in order to buy a new one. Once they are not in the active fleet, you have room in your fleet for a new ship. Second, you can keep many ships in the dry dock (inactive ships). This way, if needed, you can use them again. Once a ship is in the museum you cannot retrieve it and use it.
To see the list of ships available you have to click on the Add Ship button seen at the top of the museum window. It is not clear if you have done that. If you have click on it and still do not see your ships, close your game completely and restart it. If you still do not see them, check again they are not in your active fleet. Maybe your game is out of sync with the server and you need to get them out again.