You obviously want the very best service provider you can find to take on responsibility for your work and to do justice to your hard graft. With so many choices, particularly with the sheer amount of proofreaders on the internet, often, one of the best places to begin is in your own institution. Ask your colleagues, lecturers and tutors if they know of any editors or have used a particular local service in the past. This way you get a personal guarantee that you will be using a good service. Your colleagues may also know of good subject specific editors that will be able to really understand the content and context of your writing.
The other major area to look is, of course, the internet. Simple Google searches are good for pulling up a multitude of options to get you started. For example Proofreading provides professional document editing and proofreading services. The document is checked for proper grammar usage and flow. Look for services that represent themselves well and present their policies and services clearly. In addition, one very valuable piece of advice is to only choose companies that offer editing and proofreading reading services alone rather than in addition to writing services. Those that offer writing only offer editing to give themselves 'credibility' and may pension editing off to unreliable out sourcers. There is a very clear line between writing a thesis and editing a thesis and you don't want to jeopardise your most important academic work by being involved with an underhand company. On the positive side, however, there are lots of legitimate services on the Internet. To help you sift, look for reviews or comments in online forums or opinion pages to help you judge which services are the best.
Money is obviously a big consideration for a student, and it is fine to want to get the best deal possible. However, be aware that the better quality services often do cost that bit extra and you should look for a low risk editing service - not the cheapest one you can find.
Additionally consider their operating times in relation to how long they need to review your document and their availability: some sites are not always available whereas others will be available even at 2am in the morning - that is if you are likely to submit at that time. Finally, make sure that the editor you do choose can meet your needs, think about what you want them to concentrate on within the document such as focusing on style, ensuring an error-free document, or improving the flow of the document. It is your thesis or dissertation after all and you are employing them to help you polish your perfect final draft.