In the past, i had picked up a stereo HMV ceramic cartridge for a HMV turntable from here. Mr. Shafi seemed nice to deal with and he gave me some replacement styli as well (I think this was about 3 years ago).
Alternately, a commonly used stereo ceramic cartridge in today's applications (like portable record players) is this. Although the price seems very low and it is made in China, my experience with this type has been very good. With a good phonostage, they can produce some excellent sounds. The stylus quality is also very good. However, the ones that I had did not have a cartridge mount and I had to make one of my own. I also had to customize the mount to ensure that stylus engaged at right angles with the record groove. If you check online, you may be able to buy this one with a matching cartridge mount as well.
For using a magnetic cartridge to play music on any regular Garrard turntable or changer (assuming you are going for entry level magnetic carts) the following criteria have to be met:
1) your turntable headshell (both detachable and non detachable types) has to support either half inch mount or p-mount type carts (older Garrards have a single-screw cartridge mount, for which an adapter can be used with half inch mount possibilities (good example is the Garrard SP25 MK1 and MK2 headshell)
2) your tone arm should be able to track at between 1g and 3g (VTF) or your choice of magnetic cart may be limited to those that track at 4g+ like the Shure M3D
3) the tonearm should be wired for stereo (most Garrard tonearms are, except for the very early ones from the pure-mono era)
4) you should have an amplifier with a Magnetic phonostage (MM) or use an external phonostage
If the tonearm has a counter-weight at the rear, then 1-3gms VTF can be achieved. If the tonearm is spring balanced, then you'd not be able to achieve a consistent VTF below 7 or 8 gms. One may also need to keep in mind that the changer functionality can be impacted if you lower the VTF. The tonearm may drop and bounce or skate about on the record. Also it is not advisable to use the auto mode if you eventually install a magnetic cart as the cantilever and stylus may not be able to survive the rough and tough drops and lifts. Also stacking can cause the stylus to wear unevenly causing record groove damage, distortion and reduced sonics Essentially it is better advised to operate the changer in manual mode if you are actually able to install a magnetic cart meeting the checklist in the post above.