I recently worked on a figure for a paper with my research group where I did the exact same thing you are asking. There are no basic plant newick files out there. You can search google for newick files to published papers: I spent days searching for newick files of plant phylogenies and what I found was not appropriate for what we wanted to show. Additionally, for plants, there are few newick files including branch lengths.
In the end I constructed my own phylogeny with data from Phytozome. There are a lot of duplicated genes so I searched for genes with low copy number. I selected 41 genes from all of the plants with sequenced genomes and then concatenated them and aligned them, inspected by eye, and made a phylogenetic tree. Basic phylogenetics. This ended up being much faster than the time I spent searching for the perfect newick file on the web.
You want a subset of that big phylogeny? I don't quite understand your question. But if you're asking how to show a specific clade within the phylogeny of flowering plants, after you construct (or select) a phylogeny, simply select those branches in a tree viewing program like TreeView or (my favorite) FigTree.
If you're adventurous and in a hurry and see exactly what you want on the web, you can try TreeSnatcher, but I'm not a fan (at least I think the learning curve is high, which is a little ridiculous for a program whose intent is to be a shortcut).
There are multiple large consortia with funding to resolve plant relationships. Don't just download the raw data and try to construct a tree because these groups have already gone through all the computational and statistical hurdles to do that. If you just want to create a small phylogram as a figure, that is different.
There was a recent phylogeny of green plants published using a supermatrix approach and you can download all the data (i.e., matrices, alignments, etc.) from the BigPlant website. This is just one study though, and you can find more trees from Dryad, TreeBASE, The Open Tree of Life, in addition to the other sources mentioned. The best place to start would be to look at the paper related to that first link and follow their citations to find other large-scale plant phylogenetic studies.