It is one of the commandments -- thou shalt store opened tobacco properly.
It is an abomination to allow good tobacco to be ruined. Tins don't re-seal very well: Mason jars or mylar bags are the way to go. For Mason jars, 8 ounce jars normally hold a tin's worth of tobacco with no problem and are probably what you need. Some prefer the wide-mouth (jelly) jars. They are more convenient for filling your pipe, but cost a bit more than standard jars.
The hardware store or the Walmart has them. Make haste.