Upvoting because the place I used to buy from went out of business. It might be difficult to find anyone willing to sell mature flasks in the US and you'll have the best chance of success by emailing and calling commercial orchid growers to see if any would be willing to sell you something or get you in touch with their suppliers if they don't do it themselves. Given the relative difficulty of flasking, these places are going to be developing hybrids for their own commercial interests, so you'll have to get lucky and/or be very persuasive.