if you want to keep butter for a long time its easy to prep yourself, just get some normal butter (not marge it has to be butter) and a glass jar with good fitting lid...
take said butter and heat in a pan on the lowest heat possibleso it melts very slowly, DO NOT stir the pan AT ALL.... when all melted pour the butter into another pan VERY VERY slowly, there should be a milkey water in the bottom of the pan, the idea is to keep this in the old pan, its basicly water salt and crud from the butter.
when you have tipped all the butter into the other pan throw the milkey water away, its no good for this, you could keep it though and put it into your cooking etc etc.
ok now the "good" butter that should if still warm be clear, this is basicly "Gee", heat this butter on a full heat untill no bubbeling are left, thats the lsat of the water being boiled off, heat it nice and hot, if it starts to burn remove from the heat straight away..
meanwhile, take your jar and wash in warm water, then in hot water, rince well with hot water, while thats happening boil the kettle, once the final rince is done will it up with the boiling water, place the lid on top and turn it upside down in the sink, this makes sure the lid is sterile...
now when the butter is ready empty the jar, make sure all the water is out and the jar is still hot, tip the butter in and place the lid on straight away, leave it on the side to cool, this will cause a vacume as it cools sealing the jar....
jobs a fish, butter for years to come.... and it cost nothing but some time an old jar and a packet or 2 of butter...