The best source for potential resellers is the websites of other vendors with complementary or competitive technology.
Large vendors, such as Microsoft, have keyword searchable databases that contain info on many of their partners. If you identify other vendors that have complementary technology, you can look for their resellers and start building a database.
A good place to find complementary vendors, or competitive vendors if you want to approach their partners, is Capterra. It is an on-line directory of software solutions that is free to use if you are looking for vendors. You can do keyword searches for specific applications, for example field services management, or for specific verticals, such as retail.
If you are looking for partners in the Microsoft ecosystem, Pinpoint is a great resource. It was originally designed as a customer-facing website, but it has a sophisticated search capability that lets you find partners by geography, vertical and competencies.
LinkedIn is a great recruitment tool. Discussion groups are a good place to find the people you want to contact, but don’t be “salesy” in the group. The key is to participate and provide value, something that will encourage the members to engage with you.
For broader based technologies in bigger markets, you will likely need 150-to-200 qualified prospects on the long list.
This might sound like a lot, but if you are pursuing a market on a strategic basis, you will want to identify as many good prospects as possible to make sure you don’t overlook someone who should be on the list.
You will find most prospects aren’t interested, don’t have the time, or don’t have the budget to take on a new product. The drop-out rate is going to be high, so if you start with only 10-to-15 companies on the list you could end up without a good partner.