Welcome to the Forum Sarah,
Al-ko chassis parts are available at lots of caravan retailers and e bay dealers, There are specialists such as this link who can supply new identical or upgrades steady legs,
Are you sure the whole unit needs replacing? Alko advertise replacement parts like spindles and worm blocks - and there are plenty of YouTube videos on servicing and repairs.
The steadies are normally robust provided they are only used to steady the van once it has been levelled, although I have seen owners trying to use them as jacks to level the van on the pitch!
Not sure where you with the task but noticed that Facebook Marketplace has Pair of 23 inch steadies (new) near Milton Keynes for sale if that was of any interest or sufficiently local / came up under ‘caravan assessories’ search.