Hello and welcome
To be honest, I wouldn't bother trying to find them - because while it was a very popular cage for a long time - these days it's seen as a bit small for hamsters. Now five years ago it was seen as fine for a dwarf hamster - but a bit small for a Syrian. And so it's not necessarily a bad cage and certainly not "very" small. And it can be made to be a lovely habitat. All I'm thinking is it might be better to spend any money on upgrading rather than spare parts for it
I used the near identical cage myself in the past (Savic Mickey 2XL which was basically the same cage as the Hamster Heaven but had narrower bar spacing and different contents and no external tubes or holes for tubes). And even back then, I gave up on the plastic platforms and ladders and the litte pod houses (I still have one of the little pod houses that I use in a pet carrier
).
The ladder broke. The platform was a weird shape (my hamster's toilet was on the platform as he chose to wee in the little yellow pot in the corner of the platform, so I put his toilet on top of it so he used the toilet instead! Anyway he ended up aquaplaning down the platform as it was such a weird shape.
I then decided to get him a large house with a flat roof, to double up as a platform - so he had a nice dark hide big enough for his large nest and hoards and it fitted his toilet inside as well - so he had an ensuite bathroom. It ended up looking like this:
That was a guinea pig house but I really wanted a "laybrinth" or multiroom house for him with separate nesting, hoarding and toiletting rooms, but one of those would have half filled the cage. So I upgraded him to a 100cm cage, got him the multiroom house and there was more space for his large Syrian items. And that ended up looking like this - with lots more bedding as well
Anyway. As I say I had a large Syrian and decided he needed more stuff and more space. And maybe you have a dwarf hamster? And people kept dwarf hamsters for years in the Savic Hamster Heaven - and it can still make a nice enriched environment. Even some Syrians were fine in it (depending on their personality) although probably needed a lot of out of cage time as well.
My main point was - you don't really need to replace the plastic shelves and so on it came with - you can start with an empty cage and add things that make it into a lovely enriched environment. However, if you liked the shelf and it worked for you and your hamster, then you can probably find spares on ebay - although you might need to buy another second hand hamster heaven to get the contents!
I don't believe they are available to buy new/separately any more - they used to be sold on Amazon. But you could find a different kind of shelf that could work maybe? Some people use these
Trixie natural living corner shelf 28cm
Or you could do something like I did and use a guinea pig house with a lift off roof that makes a nice big dark hide and doubles up as a platform. Ylou could still keep one of the little pod houses at floor level. These days it's plenty of bedding that is seen as good enrichment - so they can build their own tunnels
So I'm not saying you should upgrade the cage if the hamster is happy and settled and it works for you both - just that if you need to make some changes there are various options
You can actually get second hand savic plaza cages second hand quite inexpensively and they come with a similar plastic shelf - which might also be an option. It's about 20cm longer and a bit taller (so it allows for plenty of deep bedding) and it still has a large front opening door like the Hamster Heaven (I loved that big front door!).
Savic Plaza Hamster cage
Ideally, it would be better to upgrade the cage to a larger 100cm one though
but I did manage to get him to use it for food. He used the little sand bath that came with it for a long time until it cracked. I still have the bars and the base, but nothing else, except the little yellow pot.