If you like the bark wood ones, this one is quite good.
https://www.amazon.co.uk/gp/product/...f_rd_i=desktop
It's 21cm tall, but if you have about 15cm substrate it will only sit 6cm above the substrate, so good to hide under and you could rig up a bendy bridge ladder to climb on top of it. I have one but didn't get round to using it as Charlie's cage is quite full already. I like it.
Screw in shelves - these aren't expensive.
https://www.amazon.co.uk/Little-Frie...+friends+shelf
I bought this one recently which is a very nice quality wood shelf and not too thick or heavy - easy to screw in - also a corner shelf and they sell ladders to fit the grooves on the edge. Not the cheapest though, with the postage from Germany.
https://www.amazon.de/gp/product/B00...?ie=UTF8&psc=1
Here it is in Charlie's holiday cage (the corner one on the left. The one at the back is a ranchouse cages shelf. I prefer the German one).
Ranchouse cages on ebay also sell wood shelves in all different sizes, or shelf packs like this one. I like the coloured bendy bridge that screws to the bars.
This is the pack I got
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Bendy bridge shelf
http://www.ebay.co.uk/itm/BRIDGE-HOU...sAAOSw5VFWHRqJ