You are very very early mate
but that just means you're eager
Tickets are best bought as early as possible of course. You can buy tickets in Farini and lottomatica stores but to be extra sure you can go to Via Calderini to the official Lazio store and get the tickets there. Last April distinti tickets were almost sold out the day before the match so if you can do it beforehand. I've no experience with Listicket.
About the derbymeeting I'm sure you'll be more than welcome - no need for ticket stubs, just a Lazio scarf and a friendly attitude! Hope to see you there, and come around to the forum some time before the game and you'll get all info in the dedicated topic.
"One love, One Lazio SUPER TECH!!!" - mr douglas