Also, think about a logical order for the modifications. Everyone wants to do power mods first, but if the factory clutch can't handle more power, start there. If you're doing the clutch and also plan to include a limited-slip differential, then do them at the same time to limit labor. Think about what has to be disassembled every time a part is installed and figure out if other parts you plan on replacing are involved.
Everyone wants parts at a discount or even for free. Regardless of whether you're going to build the coolest, most cutting-edge version of whatever kind of car you've chosen, a shop or manufacturer is going to need to justify any discount. Have a plan before approaching anyone for sponsorship. If it's a racecar, commit to a schedule of races and demonstrate that you're going to be more than a back marker. If possible, show evidence of past success in other classes.
The same holds true for building a show car. Outline a show schedule you'll be obligated to keep, provide examples of cars you've built in the past. If this is your first project and have nothing to show, resign yourself to probably paying retail price for everything.
Yes, almost all shop owners are enthusiasts by nature, but they're business people by necessity. It's all about the bottom line. You may be the most active user on a website and all your friends ask you what to buy, but if you can't demonstrate that, it won't matter. Giving/discounting parts or labor needs to be an investment for the business. If a place isn't guaranteed a return, sponsorship isn't going to make sense.
Last, make sure you're building the project for the right reasons. Is this a car you're going to enjoy once it's finished? Many enthusiasts have built cars and decided to sell them within a few months of completion. This isn't necessarily a bad thing, but realize that you may never get all your money back from a project car. Few people are interested in how much you spent in long-distance calls and shipping to get that South African-market spark plug wire holder no one else in this country has. Some might think it's cool, but most will only be willing to give a small fraction of what it cost.
Hopefully, this hasn't scared you from building a project car, because there's something truly satisfying about being able to finally drive your vision. Seeing something through from start to finish is always fun, but when the end product is the car of your dreams, the reward can last for years.
There are thousands of enthusiasts who regret never completing or even starting the project of their dreams. There are few who ever regret finishing one.