You can always make your own if need be - some thin (3 mm) mdf is relatively cheap (~£5 for maybe 5-10 sheets) which you can buy online in A4 sheets. All you then need is to buy magnetic sheeting, again relatively cheap, from somewhere online. Besides being cheaper, you can make any size of tray you need, when you need it. This can also be handy for making extra bases for larger models if you need them.
I personally prefer to buy either the mdf trays from Warbases or the plastic ones from Tiny Worlds but for maybe £20 you can effectively make enough for an entire army.