From what I know, there are at least 6 filter holders currently in existence for the 14-24.
1. Lee SW150 (uses 150mm filters that are 2mm thick)
2. Fotodiox WonderPana (uses 168mm filters that are 4mm thick and 145mm screw ins)
3. Lucroit by Hitech (uses 165mm filters that are 1.5mm thick)
4. Progrey G-150X (uses 150mm filters that are 2mm thick)
5. Gron (uses 150mm filters that are 2mm thick)
6. TY-T150 / NiSi (uses 150mm filters that are 2mm thick)
For me personally, Lucroit and Fotodiox are not options that I will consider because of their unusual filter sizes. For now, 150mm seems to be the filter size with the most options filter-wise.