I had the same problem (48x38x28 chainset). It turned out that MF-TZ21 is actually not a 'cassette', but a 'freewheel'. Your options for that are very limited: In the UK, Raleigh is distributing a 7 speed 13-24T model for less than 10 quid. SunRace is still producing 7 speed freewheels, but the closed-spaced 12-?? model wasn't distributed in the UK - check eBay Australia maybe.

I ended upgrading to hand-built wheels with a standard Shimano 8/9/10 speed 'freehub'. Using simple spacer rings, these accept 7 speed cassettes compatible with the MF-TZ21, i.e. I got the 12-21T Shimano CS-HG50 [1] and love the resulting gearing. You might need to fine tune your rear derailleur after installation.

[1] http://www.rosebikes.co.uk/article/shimano-cs-hg-50-cassette-7-speed/aid:414549

I had the same problem (48x38x28 chainset). It turned out that MF-TZ21 is actually not a 'cassette', but a 'freewheel'. Your options for that are very limited: In the UK, Raleigh is distributing a 7 speed 13-24T model for less than 10 quid. SunRace is still producing 7 speed freewheels, but the closed-spaced 12-?? model wasn't distributed in the UK: Check eBay Australia maybe.

I ended upgrading to hand-built wheels with a standard Shimano 8/9/10 speed 'freehub'. Using simple spacer rings, these accept 7 speed cassettes compatible with the MF-TZ21, i.e. I got the 12-21T Shimano CS-HG50 and love the resulting gearing. You might need to fine tune your rear derailleur after installation.