For general purpose electronics, such as the processors, memory, etc, there is actually a fairly broad market for radiation tolerant devices. These are often used for military electronics, people who just want to be cautious, weather balloons, etc. Of course, some of these don't meet the long term radiation requirements that satellites are forced to endure, but they are available for many missions. (Source, Source) These chips are more expensive, have limited selection, but they are much cheaper than making your own.
Many missions have requirements which exceed any commercial electronics available. These usually occur in one shot things, huge cameras, new frequencies for observation, etc. These will often require special components.
What is usually done for most satellites is some sort of a combination, using radiation hardened technologies for the most sensitive parts, either off the shelf or custom made, and using radiation enhanced semiconductors, things not made to handle the rigor of space, but tolerant of a radiation. In addition, there are a a number of commercial parts which have a reasonable amount of radiation tolerance, which are often used.
As far as temperature, and pressure, in general, most electronics will work when in a vacuum. Some mechanical work occasionally has difficulties, but that is generally easier to do than custom computer chips. Most chips work over a reasonable temperature, and the satellites simply ensure that they stay in their specified ranges. Most satellites are tested in a vacuum prior to launch (Thermal Vacuum testing), where they test out extreme temperatures and vacuum pressure, to ensure they work fine, but in general, satellites are designed to keep such concerns to a minimum.
Vibration is a bit trickier, but usually the chips aren't sensitive to them as much as the legs of chips are. There are best practices to ensure that they are able to survive the vibration analysis of launch. As vibration is only a concern at launch, they design them as best they can, and test them on the ground to verify they did everything correctly. Most consumer electronics can handle some level of vibration fairly well, due to people dropping them, etc. In general, portable type chips (Cell Phones, for example) work better than stuff to plug in the wall. The same thing applies to power.
