You can buy them directly from the manufacturer.
I have one and have had mixed results. The first consideration is keeping the film in the slots. With inversion agitation one out of six seemed doomed to move, although not always causing damage. 'Gentle' agitation is easy in theory, but 1 litre of liquid still has a lot of force in it as it drains from one end of the tank to the other, and some film bases are more flexible than others. And swirl agitation suits some developers more than others, but I found this was perhaps the best way.
But the main problem I had was with choosing a developer. Some like HC110 seem to work very well, but when I get the Pyro out for doing my favourite semi stand development I got 'hot spots' forming (seen as stripes of increased density) on the negs where the negatives are nearest the ribs of the MOD54 insert. Leaving the inner slots empty is an option.
I am sure the MOD54 can work very well indeed, but it doesn't work perfectly for me, which is a shame since it is a great idea.