Sticking to the magnetile theme of the OP, my kids and I have spent the most time and most occasions playing with the mangetile marble run kits. It works so well.
I bought the magnatile-knockoff-version essentially and while not as pure montessori as the wooden blocks, they're 1/10th the price and my 4-year-old is _loving_ this kit: https://amzn.to/3MVXRXg
Beech is the cheapest of the common European hardwoods. Even through a distributor it’s only about 1400-1700 € per cubic meter in 5cm / 2” planks. For context, the cheapest construction lumber is about 300-800, oak 2500, american walnut 4000.
It’s not great compared to 10 years ago, but the last 3 years it’s been pretty stable. About 11-15 euros incl VAT per square meter at retail prices for the cheapest 3-ply 9mm and 5-ply 12mm softwood ply, brazilian import.
I dabble in furniture, which means I’m looking at baltic birch plywood, at about 40 euros per square meter of 15mm 11-ply sheets. At that pricepoint I might as well buy actual hardwood lumber.
As an aside, there's an app out there is an app for the iPad called "Cuboro Riddles" which is a "how do you make the marble go from here to there using the blocks." Given that there are multiple ways that a block can channel a marble, this is a tricky one.
... and then if you get this over into the lego domain (not as "just something to fiddle with..." it gets you into the GBC. There is a standard for how one connects to another described at https://www.greatballcontraption.com/wiki/standard ... and then at lego conventions people hook them all up. https://youtu.be/avyh-36jEqA
Looks like their site links to a good number of US stores that sell them, many mom/pop. While there may not be a store close enough to you, perhaps there's one that would ship to you.