Unless things have changed in the last couple of years, you have the choice of enrolling with someone who runs a course for the specific qualification you want ( the course fee should then include the test fee), or you can simply contact a testing centre and book yourself in for a test.
I haven't looked recently, but you should be able to find a list of the testing centres. I believe they are run by Vue and Pearson respectively, but you'd be best to check that via the microsoft website first.
Test fee's I 'think' are somewhere between £100-150. I sat mine in 2003, so the fee's will have gone up since then.
For someone with experience, simple buying one of the qualification books and studying up on the specifics it deals with, should be enough for most people to pass without too many problems.
Microsoft do their own range of study books for the various exams, although there are several other publishers as well. Most large bookshops should stock them, or you might even manage to find them in your local library.