I used a Russian 500/8 for about 1 year before I got the Sigma 170-500. The 500/8 is usually a m42 or T-mount, and adapters are readily available.
The biggest problem you will have, is trying to focus. The XSi doesn't have a large or bright VF, so manual focus will be tricky. It'll also be fairly frustrating for anything that isn't sitting still.
I would suggest saving up a bit for something better. I've seen the Sigma 170-500 go for under $500- several times on ebay... that's a pretty decent starter lens.