I think it really depends on the product you want to test. You could probably prove there is a need by using existing tools without even building a prototype. A few examples:
If you wanna start an organic baby food company: make a few samples yourself and try to sell them via facebook groups or reddit.
If you wanna start a t-shirt company with unique designs: print a few t-shirts using something like customink and open a shop on etsy.
If you wanna start an personal assistant service: build a website on Wix or Squarespace that has a mail address and perform the first tasks manually.
This probably won't apply to every idea, but I think most can be POC'd manually instead of building a working prototype.