You can use any jabber server. Transports are ussually free-for-all. It's called "Service discovery".

Some other usefull services:
MSN transport
ICQ transport
Y! transport
gadu-gadu transport
Mail transport - this allows receiving notifications and full plaintext mails and also sending them out

To get a transport you basically need to search for a server that has it. Some do some don't. But you don't need to be using the same server.