Originally Posted by masi61

To remove you need 2 chain whips: one to hold the cassette steady and the second to unthread the smallest lockring cog.
Of all the cogs that could break, I would think the 11 tooth cog would be most susceptible. You might have better luck finding a replacement for the single cog that broke - you'll have to do a lot of internet and bikeshop searching though til you score.
Finding a replacement APII 8 speed cassette is not going to be easy. This is the ONLY cassette that will work for your application. There's no reason to be even talking about any other 8 speed cassettes or what their prices are because they won't fit. These have been out of production for 15 years or more and thus are now rare. Check ebay on a regular basis - they're not going to come up very often but probably will eventually. If you give a little more information about which model of hub some BF members might be able to be of more assistance to you. Is it a mountain or road bike? The 8 speed mountain APII cassette had the outer 2 cogs threaded together. The road version only had the outer cog threaded - so there are 2 incompatible variants of the APII Powerflow cassette right there.