OK it’s an old saw this but what you can do is buy the absolute cheapest compatible shimano caliper, and get 2 sets of seals for £20 or therabouts, which you can transfer into your more expensive model. And that caliper will also come with spare pistons (and often a bad seal will have a bad piston to go with it due to trapped dirt) and a set of new pads.
So yeah, I wish you could buy Shimano seals, but it’s not a terrible issue, especially when you consider the difference in the initial outlay and the fact that you’re not that likely to have to replace seals in any brake. (I did, but only because I couldn’t face dealing with Dave Hinde for a warranty job in order to save a couple of quid!)
PS, isn’t it funny how people always want to talk about Shimano in Hope unreliability threads? 😉