You can go a eyeglasses shop which provide contact lenses. Usually, they will provide colored contact lenses. You may find purple contact lenses there. Besides, you also try to search them through online. I just search it and find some sites where provide purple contact lenses. Here are the sites: http://www.youknowit.com/online-shop/purple-contact-lenses.cfm
Now, the colored contact lenses become more and more popular as the accessory to beautify the eyes. You could go to the lenstore.co.uk to have a look. There are many colored contact lenses there with different colors. Online buying will save you a lot of money. In addition, it is convenient.
It seems that you are pretty into purple things right? So, I can recommend some places where you are able to find loads and loads of contact lenses of various types and colors. Maybe you should have a look at Alibaba, which is a fantastic e-commerce website full of contacts, just type in key words as cosmetic contact lenses, and you will be seeing millions of contacts. Hope you find what you like.
My eyes are also dark brown. I love to try several different colors of contact lenses. Purple is a great color to try, which makes your eyes very beautiful. Yet if you would like to wear the color contact lenses for working time, I suggest you can also choose a pair of dark brown ones, just for working time. It looks very natural, and makes your eyes shine too. Purple ones, you can wear them for casual time. Yet I recommend you consider some online stores. They are much cheaper, and if you are not satisfied with the color you chose, you can make refund easily. Sometimes you can get product for free. The linkages below are for your reference.