A great way to find “normal” aircraft parts is to head out to the aviation “junk” or “salvage” yards and scrounge for as many usable parts that you can find. Not everyone wants to use previously owned parts, but there are some great deals lurking in those damaged aircraft bone yards. Reputable salvage companies can help you save a few dollars by offering pre-owned parts. You can find many of the major salvage companies by looking through the classified trade papers. Ads are also a great place to locate individual owners or maintenance shops that have a few parts taking up space in the back of their hangars.
Another option would be to check out the aviation insurance claims department salvage websites. Most claims departments put the salvage out to public bid. It’s important to only bid on aircraft that have components that will work for your projects; items like engines, avionics, instruments, etc. You could end up buying an aircraft where all the parts you need are perfectly okay, but the airframe is completely destroyed. In that case, the “airframe” still belongs to you. When you make a bid, you get everything. It’s the highest bidder’s responsibility to take all the salvage with them when they pick it up. An advantage as an individual is you can often pay a little more for the salvage than a salvage company. The salvage dealer has to be able to resell the part - that’s how they make their money. As an individual, you just want the parts! But be careful, this can also make you pay way too much. Often the sum of the parts is worth more than the price of the whole aircraft! Once I was told that a rule of thumb is to start the bidding at about thirty percent of the average retail. Of course, that will change depending on the aircraft and the type of damage. Some aircraft could be complete, yet spent a week submerged in seawater. That could mean very expensive repairs to all the components.
Okay, so let’s say you haven't found a certified aircraft to harvest parts from (besides the one in the hangar next to yours). What’s another option? Start looking for parts that are aircraft designed, but not aircraft certified. An example of that would be buying a "ground" power unit engine. These engines look very similar to aircraft versions but they often don’t have dual ignitions or as heavily built crank flanges (for mounting propellers). They also have extra mounting brackets or heavier parts. Modifications are often necessary, but the purchase price can be lower. I know of three Continental radial tank engines that sold for less than $4,000. Of course, they will take some parts hunting and minor (and sometimes major) modifications. These radial engines had a clutch assemble that weighed over 300 pounds! After the clutch was removed and the crank flange modified to accept and support a propeller (and a few other changes) the engine will be ready for installation on a bi-plane.
So maybe there are not enough ground power units for all of the custom builders around. Next option would be using an auto conversion to help reduce the cost of operation and maintenance. Or maybe you can buy an experimental engine made from aircraft parts and sold by engine parts suppliers BUT assembled by you, the builder.
Save a few dollars in the original cost by scrounging, buying used, or converting the family car engine, but whatever you do remember it’s you and your family in the aircraft. Don’t cut corners just to save money. Safety and reliability is still more important.