I don't have any advice about piano stores specifically but as someone who's family is in the retail industry all I can say is brace yourself. It is really, really, really difficult to run a successful retail store. I don't know where you are but I think in the US something like 80% of new businesses close in their first year. Make sure you do your research, not just about how to get pianos but costs of operation, delivery, business management software, legal and tax considerations, basically everything you will need to do and know to actually run the business. And good luck.