I have a '10 WRX and I use the K&N # KN-140 oil filter.
Your's should be the same....
http://www.knfilters.ca/search/appsearch.aspx?type=ps&year=2012&make=YAMAHA&model=WR250X&engine=250#&&/wEXBgUIQVBQX1RZUEUFAnBzBQ5SRUZFUkVOQ0VfTkFNRQUKQVBQX1NFQVJDSAUKQVBQX0VOR0lORQUDMjUwBQlBUFBfTU9ERUwFBldSMjUwUgUIQVBQX01BS0UFBllBTUFIQQUIQVBQX1lFQVIFBDIwMTK0Je6vF94ogOAKswwLhfVNgcDWt7fZbbVj/+gKSOQzbA==