When making paper flowers there are some materials that makes life easier - but they can also be replaced with alternatives, if you cannot get hold of them.
Floral tape: Floral tape is a long, waxed strip of paper. Florists use floral tape to wrap around the stems of fresh flowers - often together with steel wire to stiffen the flower stems. I sometimes use floral tape to make stems on the paper flowers instead of green crepe paper. I like the color of the light green floral tape - and I prefer to use the wider tape because you can quickly make thicker stems with it. The advantage of the floral tape is that it gives a very smooth, wax-like surface, which suits some flowers well. The downside is that wax doesn't adhere as well as glue... and it's a bit sticky on the fingers!
You can easily use strips of green crepe paper with added tacky glue for your stems instead of floral tape.
Floral wire: Floral wire is steel wire with a thin layer of paper on it. It is also used by florists - and also for decorations on cakes! Therefore, you can often buy it on websites that sell florist supplies or cake equipment. Floral wire often has a number that indicates the thickness of the wire. Size 20 (often called "gauge" 20) is thick wire that fits well for the flower stems. Size 28 (called "gauge 28") is very thin wire that can be used inside green leaves. If you only want to buy a few sizes, I would therefore recommend buying size 20 and size 28. The color of the floral wire is not that important because you will almost always cover it with green crepe paper.
If you can't get hold of floral wire, then you can also use any normal wire instead - it can be nice to have some thick wire for stems and some thin wire for inside leaves.
Tacky Glue: The more practiced you become at making crepe paper flowers, the more you will want to use liquid glue rather than a glue gun. When I started making paper flowers I used a glue gun for most things. I actually couldn't understand anyone using only tacky glue! I thought tacky glue was way too wet and I couldn't get it to adhere. BUT eventually I found a glue that works really well: Aleene's Tacky Glue (and no! I'm not sponsored ;) ) I mostly use their original tacky glue (in a gold colored bottle), but it can also be nice to have their "fast grab" glue (in a white bottle), which is even thicker and adheres very quickly. It's nice to have the glue-bottle that can stand upside down - so it's always ready for use. You can buy a large, normal bottle (which is a bit cheaper) and refill up the small "always ready" bottle yourself.
You can of course also use glue from other brands, but sometimes you can be a little discouraged from using tacky glue if the glue you try is too watery.