For some of us building is not an option. Building such a project will take a lot of your spare time for the next couple of years. Spouses and family may not like the idea of you building an airplane and then flying with it.
Also, if you want to fly next week instead of in three or more years from now, there is no other option then to go out there and look for an aircraft to your liking. Remember that when you buy an project built by someone else you don't buy the repairman certificate or the experience the original builder has.
This means you can only do the 'normal' maintenance a pilot does as if he owned a 'certificated' aircraft. This has to be taken into consideration, as you cannot do everything a builder could do on his own project.
Maintenance will cost more, unless you are a licensed aircraft engineer or know a friendly one, who is willing to help you in return for some flying time. Plus you can do the work under his supervision!
There are a number of web sites specializing in selling and buying complete airplanes or separate parts. Make sure that you read all the fine print for placing ads on their sites and about their fee (if any) if you should sell or buy one through them. Some require you to lower your selling price if you want to get back at the home page again.
Buying an used or new aircraft is nothing to be scared off. I have done the buying and selling part a couple of times now and it helps to be patient, the process will take some time. The projects I have helped selling were usually gone within six months to a year or so, but don't be surprised if it takes longer when the economy is not booming.
I bought my last project even without seeing or flying it. Just from the brochure. That's not for everyone, though, I did went to the factory some time later and scheduled my flight. I recommend to do a test flight, a couple (at least) of them in different weather circumstances. That really helps getting to know the machine and how it handles the weather, turbulence and how comfortable the seats are on the longer trips.
Before buying any airplane, it is wise to get an instructor or safety pilot familiar with the type. Having a good checkout on the type is very important to be able to fly safe for years to come.
I have seen pilots thinking they could just fly it, without any proper training whatsoever... and finally crashing on the trip home. Really, one picked up his new airplane from the factory and never made it home. By just watching the movie 'Top Gun' is not going to make you a pilot, but then again, some of us do have a private jet.
Think beforehand what goals you have: speed (could be important for cross country trips), number of seats, baggage capacity, fuel capacity (endurance and range), engine type and power, high or low wing type. Homebuilt aircraft usually have a single engine so I'm skipping the twins here.
Make a list of aircraft fitting the goals set above. Using the Internet search engines can be very helpful finding the right machine. There are lots of websites selling these, do not forget them. Make sure to search your local area too. Experimental aircraft associations can help you find an airplane suitable to your goals. Talk with owners on how they bought their dream machine.
And last but not least: do not forget to include your spouse in this process, if you want him or her to join you on trips and maintain a good relationship at the same time too...