You can buy new ones from a caravan spares centre, or try the caravan breakers yards. If you want to try fixing it, you need some kind of sealer. Sikaflex or caraflex is usually recommended, but bathroom sealer might do the job. Personally I would take it out and tape it with gorilla tape from the underside, then seal with sealer on the right side, then refit.
For some shower trays you can get liners - basically a skin shaped to the shower tray - which can be glued in the original tray without having to remove it.
I repaired the one in my last caravan with epoxy putty. If you look on eBay, you will find some called Milliput. It is available in white, so not noticeable on a white shower tray. Apparently you can add paint to it to colour it too, but I never tried that. If you thoroughly clean the surrounding area with sand paper (I used an angle grinder around the actual crack) to ensure good adhesion, then once applied, it can be sprayed with water and smoothed out. I tried Sikaflex before the epoxy but it wasn't very successful and looked an eyesore.
When our shower tray in the caravan got cracked badly due to a blowout on the M25, we had a molud made out of fibreglass that fitted and was stuck to the underside before replacing the whole thing, half the price of having a new one made entirely. A tighter fit when putting back. The cracks were rubbed down and rubbed over with coloured sealant to match the colour of the tray.
------------- "Close your eyes and pretend it's all a dream, that's how I get by"
I've just ordered some Milliput from an eBay store. I'll try it and and try to remember to report back on how successful it was at repairing the crack on the shower tray of my Swift challenger.
I had cracks in my MH shower tray and it just wasn't feasible to remove it as it is 18 inches up the walls and the toilet is mounted thru it as well, a bit like a corner wet room (I think the MH was built around it)
Anyway it cracked due to flexing between the supports underneath so I tried using very fine fibreglass matting which duly dropped off after the next trip but since then I have had some success with clear araldite just drizzled on the cracks (OK its under a mat anyway)
Many things will seal it but what I did do was drill two 10 millimetre holes up thru the floor and pump expanding foam up under the tray