Do you mean you want to know how to reply to a thread? Or reply to an inbox message. There is a reply button on the bottom of every post, or you can just type right away in the box. Inbox messages can just be opened via the inbox button, selecting the chat you want, and doing what I stated above.
About the Transistor, I found where you can get a BC557 here. All you need to do is select the one you would like. I suggest this one here. Digikey is a very reliable source and usually delivers they're products within 48 hours. Let me know if this helped.
Hereis where you can get a BC558B transistor if you would like one though. The sad part is that you can't buy them by one at a time, you need to buy them buy the thousands. So I recommend you just get the BC557 unless somebody else can find you a source that can get you a BC558B. I'm not sure if the BC558B is compatible with this circuit though, would anyone else be able to answer his question?