There are pro's and con's to each type of car part. New parts often times come with warranties that the used may not. If your car is still under warranty I recommend checking with your dealer or a warranty specialist as replacing parts may impact your warranty. Aside from that either parts can perform the function and there are potential problems with each.
Popular Mechanics has a guide on their website entitled Junkyard 101: How to Find Cheap Car Parts. Additionally, Car Craft also has such a guide, entitled Junkyard Engine Spotter's Guide, which focuses on cheap car engine parts rather than overall car parts.
Auto Zone, Pep Boys, Goodyear, Walmart and Sears has cheap car parts. You can also go online and check up under car parts to see who sells cheap car parts as well.
If you live in the United States they will normally cost more but it also depends on the make and model American car you are comparing with. Cadillac and Lincoln parts are also not cheap.
Salvage yard
"BestPartStore" is an online store that offers cheap, quality parts for all makes and models of cars. They sell Citroen Saxo car parts, as well as car parts for other Citroen cars.
anything from 1,500 to 15,000 if u by cheap parts than it should be cost efficient
Ebay is a great spot to purchase cheap car parts over the Internet but you could try CraigsList or Amazon.
You can find plenty of good deals for BMW car accessories on amazon. You might have better luck on ebay, though. I would recommend amazon for new parts, but ebay for cheaper parts.
If you are on a budget and need to get your car back in shape, Advance Auto Parts is the store for you. We have a wide variety of auto parts for low prices.
There is no cheap and easy way to make this swap, aside from finding a donor car for parts. And why would you install a 305? If you are going to do it, install a 350. Way better and more powerful engine than the 305. If the 305 will fit then the 350 will fit. You can buy a brand new, (Not Rebuilt), 350 for less than $2,000 with a 3/36 warranty.
For the most part, you get what you pay for. It would be better to go to a professional to get any car maintence you may need, or consult a professional for any car parts you may need.
Cheap auto parts are best found at your local dealer. Even in Connecticut, there should be a dealer that can get a good deal on parts for your particular car.