Purchase a pair (2) of poles.
These poles sell at various lengths in various styles. The ones used by most competitive teams are the 28 inch pole with a shaft and a [rubber] ball at one end. The shorter ones don't have as much weight as the 28" ones, and it makes them more difficult to handle.
You COULD make your own using the wooden sticks and wooden ball handle attachments they sell at most craft stores, but I wouldn't recommend it because I've never tried it before.
If you live in the 310 area code I would suggest going to Moore's dancewear (it's off the 405 freeway @ 4068 Redondo Beach Blvd Torrance Ca 90504). If you go to Moore's, then get the "Junior" flags as opposed to the "Juvenille" ones. If you can't remember, they SHOULD be slightly longer than your arm (about 28 or 30 inches as previously stated). Each pole would price about $25 dollars (I haven't been there in a while so the price might have raised).
The Internet contains may sources to purchase such flags, but be careful! since you can't measure up the flags with you in person, you won't be sure of the quality of the poles. Here's a list of online website sources for you to purchase your poles: Star Line Baton Company's & dancecheer.net
Sewing your flag
Here's a little diagram of how I sew my flags (You should be able to click on it). The light pink lines are where you sew, the dark pink lines are the length and the light blue is what those parts are. The actual shaft of your flag would be about 20 inches, but I don't like my flags too humongous.
The type of material is a sort of nylon that is really thin. I can recognize it relatively easy because when I look closely at the fabric, you could see little squares in them, almost like a grid all over (Some people say to buy "nylon ripstop" BUT that is actually a different type of fabric than the actual material I use). But with the fabric it's not too light to have some control over the poles, but can make the "snap" sound extra loud.
When you put it on your shaft, you COULD use the velcro that comes with your poles. But I would recommend investing in electric tape. This tape is flexible and can stretch and is relatively cheap. Take off the velcro already attached to the plastic shaft and just tape the material to the shaft.
You could also use the same electric tape for taping up the tip of the flag, rather than using the plastic tip. The plastic tip tends to fall off when using it often and under lots of stress, plus the electric tape is a little softer and more available than replacing those tips.