I would just look at recent completed sales on ebay--that way you don't get in trouble on here and you get an accurate assessment of what bags are going for on there.
As for where to get a baby falabella in grey--maybe try FarFetch? When I've wanted specific colors of bags that I couldn't find at brick and mortar shops I had some luck there (although not every time). Keep in mind you may pay transaction fees or customs dues (since there's a possibility they'll be from other countries). But if you really want something it may be worth it. I had my eye on a falabella a year or so ago that I couldn't get in the states and it was worth it to me to pay a little bit more in fees.
As for where to get a baby falabella in grey--maybe try FarFetch? When I've wanted specific colors of bags that I couldn't find at brick and mortar shops I had some luck there (although not every time). Keep in mind you may pay transaction fees or customs dues (since there's a possibility they'll be from other countries). But if you really want something it may be worth it. I had my eye on a falabella a year or so ago that I couldn't get in the states and it was worth it to me to pay a little bit more in fees.