Actually, it's seasons 1 to 11 that have become available on DVD so far; except for seasons 1 and 7, they are only available through an Amazon produced on demand platform; seasons 1 and 7 were released in the traditional DVD marketplace.
Seasons 13 through 23 are available for purchase from I-Tunes with seasons 18 through 23 also available from I-Tunes in a HD format.
The only season not yet available in either format is season 12. There was a promo during one of the episodes of TAR 23 for seasons 12 and 13 for pre-order on Amazon, however, that was never followed up with other promotions, and have not become available on Amazon.
Many seasons are available for streaming through Hulu plus, but I haven't used that service and can't tell you anything able it.