Offer to buy them coffee or lunch. Be honest, telling them you're working on a system to get THEM more work, but you'd like to leverage their expertise in order to make it the best system possible. If you have a recommendation from a friend or family (ie. a roofer who fixed your neighbor's roof) mention how they were recommended because they did such a great job. Reiterate that you're just looking for feedback and information, not a sale. Offer to meet them near their office or at a worksite to make it as easy as possible for them. Try to focus first on services that are hitting their low-point of seasonality (ie. painters are busier in the summer, so try to contact them in the winter when they have the time to talk).
Like any good salesperson, you'll need to know how to talk your way from a no to a yes when talking with them.
Then focus on finding out what each of your service providers needs to know about their potential customer for them to find your service useful. For example, some electricians will only want to do (or be qualified for) general wiring/socket replacement/etc, whereas others will be licensed to hook a solar panel into the electricity grid. Some pest controllers do termite work only, others do everything under the sun. Etc.
Also, keep in mind that you'll need to weigh the amount of data you get from customers against the amount of information the service providers will want (the service providers will always want more information).
I just finished up a contract with a company in Australia which sounds like it has nearly the exact same business model (quotify.com.au). Let me know if you want to chat further about this.