Ponies are produced by Hasbro. They are just a general toy so you can purchase them from many good toy stores. Also in the UK supermarkets sell them such as ASDA, Tesco etc. The pony has to be stripped and then you can custom design your own. The extent to which you opt to strip the pony is up to you. Some people do not remove the hair, others find it difficult to paint eyes so leave the original eye paint. I personally remove everything so I have a completely blank canvas to work with.
Costing a pony can be very difficult. For example on this pony I had to purchase a white base pony, 4 different hanks of hair, clay to sculpt the columns, and the crown of Isis worn on her head is real silver. Then there are the original outlay costs of paints, paint brushes, re-hairing tools, and palettes. All this doesn't include the time spent creating the pony.
If you are interesting in creating your own pony then the perfect site is [link]