Two options mate,buy one.You can get a crappy black widow copy from the pound shop,the elastic is shite but you can always put better elastic on them,ideal for starting out and getting your aim in.Or make your own,easy to do,plenty of sites if you google them on how & what to make them from,if you start now while your young,who knows
We used to make ours out of heavy gauge metal coathangers,normal elastic bands and cut the top inch off the tongue on a pair of old shoes for the pouch,or your little brothers new ones ,dead easy to make takes about 5 minutes,great for firing small stone