The attorney for the accused is in the best position to determine whether experts are needed and if so on what subjects they will testify. Your question is too general to permit a response. Where you can find an expert depends on what kind of expert you need. What that expert will cost depends in part on her expertise, education and experience as well as the amount of work the case will require. There is really no upward limit on what an expert might cost.
The answers varies on the type of expert required. Normally you can expect to spend $125/hour for lower end experts to $700+ if you need a physician. Without further information it's rough to tell you what to expect. Talk to your attorney and they'll be able to provide you more information
Depends on the field of expertise. Are you looking for an expert in neurosurgery, or a so-called "gang expert"? Your lawyer will know how and where to look.
You've left off the biggest part of your question:
What kind of expert? Medical to dispute the findings and cause of death at autopsy? DNA? Ballistics? Identification? Fingerprint?
There are all sorts of experts, ranging in price and experience.... and credibilty.