We bought our hornbeam hedge from _pflanzmich_. Affordable and accommodating. However, I am not satisfied - the different deliveries (we ordered in 2 stages) do not match. The subsequent delivery, because some did not arrive, is planted much later in the hedge than the others and also looks different (not such a compact growth).
I bought fruit and special plants like kiwi from _Lubera_. Not because of the price, but because of the special selection.
There is supposed to be a Dutch supplier from whom I have heard many people rave.
Otherwise, I like to go to the hardware store, but for substantial and structural trees, we have various nurseries here, each specializing in something. Everything over €30 is not bought at the hardware store and similar places.
I do look out for bargains with the smaller items.