The first and probably most obvious place to look for masters dissertation help is your tutor and you'd be amazed how many students don't think of this. It's true that professors and tutors have limited amounts of time and that pinning them down outside of lectures can be difficult at times but that doesn't mean that they won't help. They know your academic strengths and weaknesses better than most and what would be a good dissertation topic to address so why not use them as much as possible.
Fellow and previous students are also a resource that you can exploit for help. These people are either going through or have gone through the same experiences that you are and have overcome the problems you may be facing. Your peers may have also found solutions to those problems that you haven't thought of. Get involved with any focus groups or study groups in your subject that are regularly discussing all issues connected to your Master's dissertation.
Another good source would be to look at what's gone before in your chosen subject. Dissertation databases can be found at your university and library either in paper form or electronic format. If you can't find something at your own university then look further afield to other universities or academic centers who specialize in your area. By seeing what's gone before you will get an idea of what's expect from you and the standard required to gain those great marks.
Finally, you could look online and employ one of the multitudes of writing services. The companies provide professional writers in your chosen field and can help you with everything from choosing a topic, formatting and editing, Abstracts, and even writing the paper for you to your specific requirements. If your writing skills are not the greatest in the world then these writers can guide you and improve your writing, as well as giving you access to resources and ideas that you may never think of on your own.
So there you have it, 4 great resources to take advantage of if you become stuck with your Master's. All you have to do now is reach out to them and make the most of them.