Replying to Balance Practice

I want to get ready for the season this year because I hardly got to ski last season. So I need to work on my balance for rails. I saw that a lot of people said slacklines were good, but I live in a cramped apartment with no backyard, so I can't do that. I also heard that standing on a foam roller helped, but my sister has back problems and took it to college with her and I can't seem to find any in fitness shops. I'm also kind of on a budget so I can't get anything fancy. Could you guys give me some advice? Thanks!