It depends what type you want.
Ifor williams do one that fits into their trailers. They are a couple of hundred pounds new!! I have one that is looking for a new home in the future (oops not advertising) It has only been used a couple of times
Other than that don't know where you find different ones
I got mine from robinsons, although a lot of people at they yard made their own out of kitchen cupboards! they actually work quite well, wish I'd thought of doing that first!
A friend of mine made one out of a Freezer! Stood it on end, ripped the gubbins out of it, fitted a lock, sprayed it and it did the job, you had to look twice to realise that it started out as a freezer that cost a fiver from the reclamation centre.
I just used an Army surplus metal locker, the only complaint was that the saddles had to go in sideways rather than the usual way.
I got the Derby House one (about 3yrs ago) but could'nt find them in the online shop, not sure if they still do them. Are Robinsons not get any more in?
Super_Kat has got a tack locker, if you PM her I'm sure she'll tell you where she got it, but she might take a while to reply as she is on work placement.
Years ago people all used to use chest freezers and put a lock on them. My dad made me a wooden tack box with a saddle rack in it. A Girl at old yard had a Robinsons one but I didnt think they were very secure. You could also try a metal filing cabinet??