One of the first things I did when adapting my house, was buy a heap of trays and baskets. Changing from a normal house to a montessori house is a process, it takes time. It's enjoyable and calming too! Especially if you, like me, love moving furniture.
You can buy expensive montessori approved trays. Yeah, cause it matters. I bought a heap of trays and baskets from the op shops and I'm glad I did. Firstly, because I love op shopping and always need a good excuse. Secondly, buying a variety had given my an opportunity to discover what I like and need without spending lots of money. Well, what I like and need is a variety! Some trays work great for one project, but not for another. For some activities I use small square trays from ikea. Other activities require large trays. Some materials fit well in round baskets, others fit well in square. So, it's a bit of searching and trying out.
One thing I noticed, though, is that consistency and plainness, works well. I have 3 yellow square trays that I use for pouring. These fit perfectly perfectly in Seb's play kitchen. Couldn't have made it better myself and it's not even an ikea play kitchen!
I got a couple of wooden serving trays with pictures on it. I don't like it, the pictures distract from the exercise. Plain color or wood is great.
I also got a glass serving tray! I love that one. It's got 3 dishes in it, so it's great for sorting, spooning, thonging and the like. I like it too, because it's not wood. It gives an extra sensory experience.
Same with the baskets. Some are woven, some are solid, some are plain, some coloured. The different textures add to the experience, but it can also be distracting. This is something to think about when buying these things, before you buy them and not afterwards, like I did.
Most of my trays and baskets are not in use. I rotate them with the materials. I take them out when needed. This is another reason to have a variety. Especially so when you're designing your space on the fly, like I'm doing!
I have no idea what Maria Montessori says about trays and baskets. So far, in my reading, I haven't come across it at all. Likely, that says something about my limited reading, though and I hope to add theory to this post one day.
In short, when collecting trays and baskets for presenting materials, keep in mind the balance between useful variety and peaceful consistency. Be prepared to buy more than strictly needed and don't feel like you have to buy new ones for 10 times the price of op shop ones.