They are on general sale(ish). Current season ticket holders have until some date in November that I can't be bothered looking up to renew their season ticket. If they choose to do so they get an extra discount. You cannot, until that date, buy a season ticket that was owned by some one last season. This is not a problem if you wish to buy one for the South or West as these are standing, so unreserved (although you may wish to check with SLIM before purchasing a West ticket, as there may be one spec that is reserved).
If you want a season ticket for the North or East, you will have to wait until said date to see if current holders have not renewed, then their seats become available. For seats that were not taken by season tickets last year, pop down to the shop, as these can be got now. This includes the new corner, although I believe you will not be allocated a seat or row at the time of purchase.
Hope this answers your question. Clear as mud.
