I help retail start-ups. 10+ Years in retail procurement/buying, assortment planning, demand planning & forecasting, replenishment and supply chain planning, store operations, cost reduction. Retailsidekicks.com
Firstly, congratulations on running such a successful business!
There is no silver bullet here. Back to basics. Sales = understanding your customer's needs and then meeting/exceeding them. If you are targeting independent stores, they will all have different needs, based on their existing strategy and constraints.
In my humble opinion, you have the option to either look for a communication channel that can reach a broad spectrum of independent store owners, or pound the pavement and try to create deep relationships with a smaller number of high potential clients.
The strategy will depend on your available resources, and whether you are looking to build new sales channels for independent products, or your entire line. Otherwise, a lower margin alternative may be to sell through a distributor to gain volume.
VC is by no means my area of expertise, however, I suggest partnering with the biggest benefactors of Uber's exit (in my opinion that would be the Taxi companies). Who are the most influential taxi company owners/corporations? Are those companies tied to larger investment firms? That would be my starting point.
Margins can vary by manufacturer depending on whether they are first (eg. Samsung TV), second (eg. Toshiba TV), or third tier (eg. RCA TV).
In your negotiations with the manufacturer, I recommend that you discuss price protection to ensure that your margin rates remain neutral as the product becomes obsolete. Alternatively though, you may be able to negotiate lower costs if you choose not to be protected; however, you would want a robust exit strategy to ensure your margins are protected should your sales not materialize.
These phone cases are not for the latest version of iphone and so therefore most of your sales will come from existing users looking to replace their current/old/broken case. As mentioned earlier, it is difficult to compete in the online space or at other mobile-phone or electronics shops.
Perhaps a different sales channel may help, like convenience stores, petro shops, or coffee shops. These are places where people are more likely to impulse shop or pick up a convenience item.