Well Inter has gone down hill the last couple of seasons. It is true that they have no fasist fans, or hoolignas, but they have very little fans currently. They are in a huge crissis the last three seasons. If things havent changed too much you could get some of their stuff at their stadium, there used to be a Fan Shop there, but things might have changed.
Then there is Artmedia, the new big boy. You can get their stuff either at their website, their stadium or at a sports shop called Atak. It isn't too far from the city centre.
What I find as their biggest problem is the people who run them. People who care little about football and fans. There is an Ultras group, which is quite calm since it gets no money from the club for coreography and so on.
Slovan is the oldest and biggest club. The support has once again been growing, since we are headed for promotion finaly. But the problem would be that there is a main Hooligan, quite far right group. It is something that gets me quite upset, but i tend to bite through it. But there are many people in the ultras group who arent some far right, and tend to act as a counter weight against the Hooligans.
But Slovan is the bigboy, that shall once again rule Slovakia
You should be able to get their shirts at Atak and maybee at the stadium. There is a online shop at www.slovanfutbal.com
but it itsn't qutie functional yet.
The main problem in Slovakia, is that merchandising is at a very low level, but you should be able to get some stuff... If you need any help, more questions are welcome.