You can buy a recoloured version from the PVP trophy crystal vendor.

If you need the original version, I think you can get seals from A and S ranks without having officially unlocked Hunts.

Beyond that, you can sometimes get them from retainer quick ventures, but that's not really viable.

But really it doesn't take that long to get yourself ranked up in your GC – focus on doing the daily levelling roulette as the role in need, get through the hunting logs and run a few dungeons unsynched. Once you get there, having access to B rank "wanted posters" is your quickest and most reliable way of getting a chunk of seals for minimal effort when you want to buy gear from the vendors. There are multiple unique gearsets and other prizes from each expansion.