Since I buy them 15 at a time I buy them at the local feed store in OH. but I know they have them locally because I turned some PA. guys on to them.
Just look for the brown blocks with the yellow label on them like in the pic. below.
I dont know all the trace minerals, salt etc. that are in them because I dont use them to try to help the deer grow bigger racks, bodies etc. because mineral blocks, licks etc. are useless when it comes to that,
I just use them to get them in front of the camera.
As far as how long they last , I put new blocks out last July and most of them should still be good until I replace them this July, depending on how much they hit them this spring, early summer.
( I just checked many of them when I was out shed hunting last weekend.)
The other thing I found to work really good other than Apples like you mentioned is Buck Bran that you can get from Cabelas, the deer will tear it up, my buddy put 25lbs out of it last friday and it was all gone the next day, but it is alot more expensive than the generic blocks, but I would say it works better than anything else I have come across. Pike