There are parts out there but the good ones are hard to come across and not cheap... When an airplane is scrapped it is essentially stripped of all useful material for as much money as possible, all the metal, aluminum, copper etc is all removed. All of the avionics components and parts that can be used as spares for other airplanes are removed and re-purposed as well. Engines, nacelles, nose cones, rudders, doors etc. are usually taken off and reused to service other airplanes in the fleet.
I work with all these airplane parts every day and it is very rare for avionic components or other "repairable" parts to be scraped unless they are beyond repair. In that case it is company policy (and is usually an FAA/EASA/Transport Canada etc policy) to remove all the plates/tags showing the part number and serial number and to physically damage/destroy the unit to prevent it from finding its way back onto an aircraft. Black market aircraft parts is a big issue in some of the still developing countries and can be a huge risk to aviation. I have personally smashed a few avionic components with great joy, it's a great way to release some stress on something you know is worth more than your yearly salary!
For example the airline I work for went through a seat modification a few years ago and all of the old seats were stripped down, PN plates removed or scratched out, all the arm rests, seat cushions/covers, belts were taken off and put back into stock and the remaining aluminum from the chairs was cut into manageable pieces and taken to the scrap yard. The money we got from the aluminum was then used to buy all the employees pizza for lunch.
The best bet for some parts would be off an older aircraft, like the 747, many have been scrapped and there are more parts available for purchase then say the much newer 777. Ebay would be a good start, I did a quick search and found a few items although not many of them were under $100. There is some 747 yokes that were listed for $900 each and presumably an item that large and heavy would come with a hefty shipping cost. Nose cone's, forget it, they are almost always taken off as they are interchangeable, not to mention that a 737 nosecone alone is worth $125,000 and almost 6' tall. There is definitely a few cool pieces for sale but finding a section of fuselage or a throttle quadrant would be extremely pricey as the only way to get your hands on something like that would be via someone who has an inside advantage.