You're best bet is to buy a new one. Most loose vintage figures can be had for $5-10, super mint ones may go up to $20 or so for most. There are exceptions like last 17 and some of the original 12 but for the most part they are attainable in that range.
If these are childhood figures and you want to keep them but just hope to display them better just do what I did as a child, wrap some thread around the joint. It tightens it up enough to stand or hold an arm out.