I too am a bit of a Normandy campaign nut and have built a few small vignettes to date. Bocage terrain is a tricky one as there is so much vegitation required. This is a nice example by Hiroyuki Kimura on FB
MK35 Editions are a good source of pre-made Normandy dio materials. Sold via FOG models they also sell some great diorama models and materials. I have many of their plaster buildings yet to incorporate into vignettes.
https://fieldsofglorymodels.co.uk/collections/all?constraint=brand_mk35-editions#
As for modelling materials Sculptamold is a very accessible product for creating groundwork. It is essentially an expensive papier Mache. It does however come in dry form, so you add your own water mix depending on what you are creating. As already mentioned, adding some cheap artists acrylics to the mix will avoid the white spot syndrome of having to paint it after laying it down. Working in thin layers works best to speed up drying but is very flexible.
Das Clay is another quality product that you can use for creating groundworks.
You are in the right trade when it comes to base material as picture frames make excellent bases. I prefer the deep box frames myself as you get more inner surface area to attach the sides to if you like to raise the ground level, which always looks better presented IMO.
Balsa wood is also relatively cheap and easy to work with material. Great for making accessories framing vignettes and dioramas. Plastic card comes in all sorts of wonderful shapes and sizes. From thin sheets to complex profiles, it is a fantastic material to work with although decent ‘choppers’ to cut the card true and straight are expensive.
As far as vegetation goes, plenty of natural material can be utilised and preserving plants with Glycerine is a great way of keeping costs down. Just be aware of the scale effects when using them.
There are many manmade vegetation and fauna products for sale. Check out any quality model retailer’s diorama section for what is on offer. Grass mats are great. Static grass the railroad modellers favourite works well, although to avoid the flat appearance of laying it by hand a flocking machine of some sort will be required to give the grass a more natural appearance.
Styrofoam is a quality product that can be sculpted and manipulated to create buildings and there are many YouTube vids on the tinterweb that show what a versatile modelling material it is. In the right hands there are many amazing creations out there. Foamboard is another product that many use to frame scale buildings
As for adhesives PVA glue for pretty much most things, Gator Glue makes some great products and UHU working best Styrofoam product.
Many of the mainstream modelling brands offer terrain surfacing products like mud and water etc.
Since I started creating more and more scenes for my models you get to find the inspirational modellers out there of whom some solely build dioramas. Take Per Olav Lund for instance. The guy is a miniature master
https://www.facebook.com/per.o.lund
There is a guy who I found on Facebook who makes the most realistic 1/72 & 1/35 scale trees I have ever seen. They are not cheap but then he hand builds every one he makes. If you don’t have the time or inclination to make a tree yourself, check this guy out
http://drzewamodelarstwo.pl/en/
Finally, if you haven’t already heard of him, please watch this modelling legend work his magic and tell me you’re not impressed. Apparently, it was his first attempt