Europoint have an abundance of samples at the trade counter in bellshill with foamex, ACM and the acrylics range they do so that gives a good bit of wall display for you.
I’m sure if you have a chat with who your using for rollup banners and flags etc they will have spares or rejects lying about. Some rejected stuff can be a spelling mistake or such so still a perfectly good option for display.
I think companies like signgeer have quite an extensive catalogue with POS and every other thing they stock so these are handy also.
One thing I would say though, flags and roll up banner materials vary so much that you can tell the difference in quality so make sure what you have on display is what your selling. It took us a while to find the roll up banner material we use now and have customers sometimes asking "it’s still the same material isn’t it". Flags have different levels or perforations and meshes and they can all look totally different so get a few if possible.