Are the buttons actually damaged or is it the thread holding the buttons that has come loose? If it's the thread, a cheap sewing kit with a similar-colored thread will do the job to tighten/secure the buttons. I know those plastic buttons can be flimsy and the "male" tab is known to break off. If a broken button is the case and you want it as close to "stock" as possible, you can find comparable replacement buttons at a craft store or online. As was mentioned above, you can use double-sided tape or even the smallest amount poster tack for a quick, less permanent fix. I remember some people were also removing the stock buttons anyway and replacing them with tiny craft magnets glued in their place, getting rid of the profile created by the bulky buttons and still allowing the jacket to close (with a slightly better look/fit).