Finding developer(s) for a big eCommerce project?

I am a front-end developer and have been managing a couple eCommerce sites the past 10 years.

Recently we purchased high-quality printing equipment (sublimation printers, etc.) so we can do print on demand fulfillment. After a few months of doing this with a simple product customizer on our own site (just add image and text to a single t-shirt, etc.) and improving our workflow, we want to do something new.

We want to build software similar to Printful (https://www.printful.com/) where we are the company like Printful who fulfills the orders for our customers. So businesses could add our plugin to their website (like Shopify, WooCommerce, etc.) and we would fulfill the orders to their customers and they would get to set the price after their base cost from us.

After doing some research this week it seems like a huge project and these companies have teams of tens or hundreds (not sure how many people they started with though)

I don't have much experience with something like this so I am not sure where to go from here. We have a decent budget for the project, but not sure the best way to find developers who we can work with to make this a reality. We've used UpWork in the past for our previous customizer, but it was very simple compared to this.

Any ideas? Thank you!