Well, I'm not from Lisbon and have only been in the city a few times, so I don't think I'll be a great help. Anyway:
1) Well, I think the best bet would be the center of the city, probably the downtown, but to be honest I don't really know, someone who lives in the city will be able to clarify this I think.
2) You can buy them either in the stadium or through Sporting's
official site (it looks like the english version is unavailable though) as far as I know. Oh, I've checked the fixture list and apparently there won't be any league matches.
3) Well, I've never been to a Sporting match (thank God) but I don't think safety is an issue there. I'm pretty sure you won't find any problems if you eventually go there. A trip to Benfica's stadium would be a better idea though.
4) There are quite a lot of interesting places worth visiting. There's the
Park of Nations and specifically the
Lisbon Oceanarium. That's a must go in my opinion. Then, if you like history you can visit the
Castle of Saint George, the
Belém Tower and the
Padrão dos Descobrimentos as well as the whole
Pombalin Downtown. Lisbon is a really beautiful city, there are lots of interesting places not only in the city itself but also in near places like
Sintra, for instance.
Hopefully someone who lives in Lisbon or has been there more often will be able to clarify all this. Anyway, if you have any more questions I'll be glad to try and help you.
Have fun there.
