Alternative to train from Poznan to Berlin

[mustobe]

Dear travelers,

I have bought my train ticket online (from Berlin to Poznan) but on the day I need to return, the trains are full (at least is what DB tells me on the website).

I need a bus to Berlin from Pozna on a Sunday, can you please advise me which website?
I am looking but found no website yet.
I tried Reisebus and came out with an empty page for the lines doing Berlin-Poznan-Berlin.

Anyone there that might be able to help me with a bus website I can buy my ticket on?

Thank you very much