I charge non-refundable deposits on every property. We typically have one shot with one buyer to close. If you can't perform, I'm not happy but at least I get my EM back. I won't take that loss if you get cold feet, didn't understand your construction numbers or can't secure funding.
With that being said, wholesale properties are for seasoned investors. They know what they're looking for, understand their construction pricing and can assume the risks. New investors may want to work with a realtor on their first deal. The contracts and structure will give you more protection.
As for the construction contacts, if they're unwilling to share contacts at RE meet ups, go to Angie's list and make some calls, walk their projects and check their references. Understand what they're charging per sq ft so you are more precise when running your numbers online. It will certainly help with the speed of your offers. Try and limit your first deal to a cosmetic flip or hold.