Where can I get medium moster bones( I need four more) and can I play online..

Do 'the great jungle kut-ku' quest and hopefully after the mission you will get some bones

Perhaps they might build an extension pack for the game that can be stored on memory card of your psp, but no not at the moment certainly.
Med Monster bones can be found on any gathering mission in the desert, as you go into not the first big sand area but the second, or it might be the first, walk along the left side of the wall and you will see a small entrance into the rock, go inside and you will see the big almost tortoise like creatures inside kill them and carve them up and eventually you will get one, I have done it myself.
Hope this answers you question

You can get med. Monster bones by killing the Yian Kut-Ku and in the rewards screen there should be some. Also if you go to a gathering swamp quest in the gathering hall there are a couple caves in which you can mine for earth crystals (make sure to bring a pickaxe) and if you go to the forest and hills, go to area seven and talk to the little man. He will trade you some med. Monster bones for your earth crystals, rarely he will give you a bone. And no Monster hunter freedom is not an online game, only ad-hoc.

You get the from killing a yian kut-ku as a reward or if you kill an aceparos and carve it up sometimes you get one