I've looked around for a while but I've not been able to find any specific plugins/add-ons that would allow me to do so. I could be missing something but I feel like everything is pointing toward having to get it custom built.
Yes, you can do it. For example, if your main site is xyz.com, then you can create many subdomains like a1.xyz.com, a2.xyz.xom etc. Please choose the right hosting company for you who would allow you to create as many as you want to as they restrict it artificially.
You can find multi-websites feature available on most popular ecommerce framework .
What's your ecommerce platform?
I would go about this by first creating sub-domains (Very easy on CPanel) and then installing the independent Stores each on it's own sub-domain. The main store will not be affected in any way. You can then use links as you would an external site to link to your other stores. No need for a plugin there.
As others have noted, most popular ecommerce platforms (Shopify, Magento, etc.) will let your site host multiple storefronts/suppliers. However, fully integrating your suppliers to list their merchandise and manage real-time inventory levels, product data, and transactions is a separate matter. Most online retailers use a SaaS like RevCascade to operate their multi-vendor online marketplaces. Happy to share more if you're interested!
There are many platforms out there that support multiple storefronts. Magento is just one of many which can get you what you are asking for:
I'm not sure I understand your needs.
Do you want to sell multiple different items that are unrelated to each other but combine their back end (credit card processing / inventory control)
Do you want to sell the same item, but in different ways, on the same site.
If the former, why? If the latter, you should be thinking about this in terms of landing pages, not storefronts, imo.
If you have Wordpress, a dirty workaround would be to create some main categories representing each store.
Then in each store pages you filter categories as being children of the main category that defines the store.
That can be the cheapest way.
I too believe that it needs to be custom built.