As mentioned you can enter yourself as a private candidate. Try the likes of AQA or Edexcell websites, they have all the relevant information. All GCSEs are changing to a linear format, and maths was one of the first to do so. If you decide to do it via a private entry, it just basically means you will do two papers. A calculator and non-calculator, as mentioned its pretty cheap to do, although have a search around local colleges.
if your confident enough to go down the private candidate route, I would recommend using textbooks, you can normally get a textbook specific to the awarding body and format e.g AQA Linear (foundation/higher). Although if you feel like you need the extra help opt for the course!
Good luck!