Every company is a little different. But if your selling solar panels for example, try googling "Solar panel installation" and then find a company that can do installation across the entire U.S. Call them and close them just like a supplier. Explain that when a customer purchases from your website, you will direct the customer to them afterward and the customer can book the installation appointment with them. They charge the customer individually for the service, So you don't have to hire anybody.
It's not easy. Expect most of them, like 95% or more of them to say no. Also, they will ask questions about your return policy, and installation (if you're doing big items). You are going to have to figure out how to negate risk for them. For most products that require installation, for example, solar panels, the companies don't make any money on the installation part of the business, so they are not going to want to work with you if you're going to be driving sales, but not providing installment, because it will be cutting into their margins. This was a big hurdle for me personally. I had to partner with another 3rd party installation company to ensure I could provide installation for my customers. Once I was able to tell my suppliers "I can take care of the installment, don't worry about it" they suddenly opened up.