Purchase Order to supplier

Hi,

It seems that you're looking for the opposite of a shopping cart solution.
In a shopping cart solution (I mean by that an shopping cart for online stores, like HikaShop and most other known ecommerces solutions out there), you normally have many entities (usually, customers) requesting goods (and usually paying for them during the requesting process) from one entity (usually the shop owner, yourself).
From what I understand, what you want is the opposite:
One entity (yourself), requesting goods (as a PO) to many entities (the suppliers).

So to make it short, HikaShop can't do that as it works the otherway around, and it will be the same with most ecommerce solutions for online stores (shopping cart) that you'll find. So no, it's not an essential part of any ecommerce solution, which is why you have a hard time finding something for you on Joomla.
What you need is something else. I'm not an expert in the domain but I would say something like an ERP. It can sometimes be categorized as "ecommerce solution", but usually it's much more than that.
You'll find ones which work online and can handle what you're talking about.
For Joomla, the only one I could find which might be close to what you want is:
extensions.joomla.org/extension/quipu-erp
Other than that, search for "suppliers ERP online" on google and I'm sure you'll find plenty of solutions you can setup to do what you want.