I would suggest you think about what your needs are first. POS systems can be anything from a cash register to a full-fleged system that includes POS machines, web-store, club software, complance etc. Martin Olsen with Napa Valley POS might be a good place to start. Good luck!
Elypsis in Napa is a well known name and has many quality products. We were actually Microsoft's POS Partner of The Year this year. Please let me know if I can answer any questions
I work for a family-owned winery and spend month researching POS systems. For smaller wineries the options are tricky and since most wineries do not have in house IT staff, you really need something turn-key and inexpensive to begin but it also needs to be able to export all your data as you grow. The support issue is also an area you want to look at as many groups have either limited or very expensive support.
If you want to chat offline, I'm happy to share what we learned. Just let me know.