I'm new here, so I'll play along.
How you finance a prototype has a lot to do with what you plan to do with it when it's done. Well, more accurately, is has to do with how risky the business is that will presumably sell the product once it goes into production.
If you have a waiting list of buyers for your finished product, then building the prototype is a little less risky. After all, you already know that people are going to buy it when it goes into production, so you might decide to get a bank loan, if you have the assets and credit and everything else required to secure this financing.
However, most prototypes are for products where you're still not sure exactly how much it will cost to build the final product or whether anyone will actually buy the thing when it's done, so you're probably looking for pure risk capital, not debt. It would not be wise to borrow money for this sort of venture, even if it were available to you, unless you are really willing to gamble that the business will fail and you'll still have to repay the loan.
Before you get to the prototype phase, you need to have a strong plan for what you'll do with the prototype once you have it. Where do you envision having the product manufactured? What might it cost, more or less, to make? And most importantly, who is going to buy it when it's produced, and how will it get sold? Are you going to build a business around the product, or are you going to sell the whole thing to another company and let them grow it?
All of these questions need to be addressed in a business plan, and then you can take that business plan to find the right kind of investor for your company. Phase I of your business plan will be securing funding for the prototype, and you'll probably be best suited to looking for a traditional angel investor as opposed to a venture capital shop.
It sounds like you're not exactly forthcoming with details of your product. I don't necessarily fault you for that, but you have to know that on forums such as these, you have to give information to get information, otherwise we're dealing with a vacuum, and it's borderline impossible to give relevant, actionable advice.
Feel free to contact me offline if you'd like to share more details that you can not share here. Perhaps I can point you in the right direction.
