I bought the Heliopan 10 stopper from ebay, think it was £40. I have read good reviews about Heliopan but they will never equal the likes of Lee filters.
The filter size is sometimes printed inside the front of the lens but a lot of newer lenses don't have that. Either check out the manufacturer website and look for your specific lens or pop on amazon and check the specs of your lens, this will have the filter size.