You will not sound desperate (and so what if you do?). I have been through this a thousand times (literally!), better to call untill they get tired of you than to give up because you want to come across as polite. How do you know when they are tired of you? They will tell you! When they do, you ask them when a good time would be to call again. If they say "never!", call back 3-6 months later, and say that you have been in correspondence earlier this year, and you can see that there was some interest, so you wanted to catch up on the matter (of your product in their stores). I have called more than 1000 potential clients in the last 9 months, and 2 of them has been harsh over the phone, everyone else has been really nice and helpful.