You wouldn't think it would be hard to find a good transcriptionist today with the economy the way it is and so many people desperately looking for work. However, there's a lot more to finding a transcriptionist than just Google-ing for one.
First of all, there's the matter of training and experience. Not every transcriptionist has had the right kind of training to be able to do your job correctly and quickly. At the very least, your transcriptionist should have a Bachelor's degree as well as some type of special training in English grammar and spelling or medical transcription training which teaches all of the above and more. Grammar is not taught well in many schools these days, so don't naturally assume your potential transcriptionist can spell, type, or even speak English correctly. Ask for a resume or CV, and check out the person's references.
Then there's the matter of speed. Not everyone types at the same speed or has the same error free result that you would like. I personally only type 75 corrected words per minute (CWPM removes any typos before the speed is calculated). That's not as fast as many transcriptionists, but I am also keenly aware of errors and typos and try to avoid them the very first time around if possible (hence, the slightly slower speed). Just because someone can hit the keys at 100 words per minute doesn't mean you'll be able to read what they typed when they're done! Whether or not they proofread their documents is also crucial and something to ask about when looking for a good transcriptionist.
Along with the typing speed, you need to consider turn around time. It costs more to get your documents in 12-24 hours. So perhaps you could plan ahead on your audio or video files and then allow your transcriptionist a little more time to work on them (like 2-3 days, or 4-6 days). That would be significantly cheaper for you.
The last consideration when choosing a transcriptionist is, are they agreeable or disagreeable? This is very important consideration because, believe you me, there are some very disagreeable transcriptionists out there! They can be some of the most grouchy and touchy people around. Part of the reason for that is transcription is an intense job if you do it full-time. Put yourself in their shoes. Sitting at a desk in some corner typing as fast as your sore fingers and carpal tunnel syndrome can go for 10 or 12 hours a day does not a happy person make. That's why I would suggest that you conduct some correspondence with your transcriptionist before hiring them and try to ferret out their personality traits, true feelings, and work ethic. It could save you a lot of trouble in the long run. One of the reasons I only do transcription part-time is because I want to continue to be a happy transcriptionist with smiling satisfied customers.