Hi there! There are a lots of plants that you can pick up relatively inexpensively that will fill up your cage quickly! My favorite plan to use is Schefflera Amate. You can sometimes find them in stores, but I order mine online from Home Depot. I get a 10" bucket with two little trees. I separate them and spread them out as they will grow quickly with decent lighting and water. I think I paid $24 for a 10" pot, so figure $12 per plant, and one plant that has had a few months to grow can easily filll a 2x2 cage!
Another good one is Schefflera Arboricola. This is usually sold as 3 short little tree stubs ina 10" pot. Once again, separate them and spread them out and they will grow as little trees to make your cage look like a little forest. I believe this is also around $25 for a 10" pot, which could then be broken up to make 3 plants.
Both of these don't have great "branches," so you'll still want to use sticks of various sizes from outside. Some of mine had minor mold or fungus blooms, but that's normal so long as it dries up and fades. You can also bake them (carefully!!!) to kill any insects, fungus, or bacteria currently growing on them. I usually just wash mine really well with the hose, or diluted bleach spray and rinsed super well if I am concerned about the branch.
Cork bark is a great addition - my guys love climbing on it and it adds a bit of dimension to the cage.
Pothos and philodendron are popular plants used, often in the upper part of the cage. Pothos is especially cheap, because you can easily propagate it to make larger or separate plants! I never seem to do well with pothos in the cages, personally, but my pothos do well outside the cage. Go figure! I LOVE using climbing philodendrons in the bottom of the cage. They grow like weeds and will climb the branches or cork bark. Another easily propagated plant, and they do really well in the cham cage conditions.
