I've always used plants from dollar stores, walmart, whatever. I used them with angels when I bred, and use them with my bettas. There are a few things you have to keep in mind, but for the most part, they can be totally safe.
Clean clean clean. No bleach, but vinegar is fine. And you want to stay away from scented fake flowers. Boil anything you plan on putting in your tank and check for bleeding. And remove anything that might flake off into the water. Get a good exacto knife and go over each plant, cut off and smooth out any barbs, remove points and stocking test. Some plants you might want to stand, and you can leave the wire in if the sheath around it is in tact and you put sealant at the end where the metal is showing.
You have to be 10 times more careful picking flowers, cleaning them and making sure they're safe, but cost to material, it's worth it for some people (like me)
If you're going to glue anything, use aquarium grade sealant and follow the directions to the letter.