Hey Kaail,
Having a mentor or trainer in game is awesome development for the Weaponmaster class and can be a lot of fun too! But no, it's not required--there are a lot of ways to achieve this PrC. Your character could hone and train and practice in solitude to the peak of perfection and skill in his chosen weapon ... but you'd still have to show some development supporting this, usually in the form of logs (even if you're just RPing with yourself basically!), CDT entries, or some other similar means.
Hope this answers your question. If not, please let us know. We're here to help!
~Carillon