Exercise bands were the best slingshot band material around not so long ago, and some still use them. Although not all are created equal. Theraband exercise bands are the most popular. That said, the slingshot specific latex you can buy from slingshot suppliers truly is better. Not twice as good, but measurably better. And for $20 or less you'll end up with enough latex for a year's worth of bands. Or, if you want to get pre-made bands a lot of suppliers sell those as well. A bit more expensive but still pretty cheap overall in the end. Simpleshot and Sling-tech are two a good US based suppliers that sell quality latex, ship fast, and have good service. And if you are willing to gamble a bit on quality, Amazon has many suppliers that will send you a bunch of pre-made bands at a great price, though the quality you get may vary. Many other good ones on the web as well though shipping costs and times may go up for those that are farther away- including some really good stuff from Chinese suppliers that can take a month or two to arrive.
If you want to get going quickly, I think several on the forum regularly use exercise bands available at local big box stores and they could probably tell you what to get and how to set it up. You could be shooting those by this afternoon if they are in stock.
You could also run through the sporting goods section of the same big box store and grab a daisy slingshot and some replacement tubes and be shooting by this afternoon for $20 or less. These won't be the pinnacle of performance but will shoot about as well as anything did 20 or 30 years ago. You could start getting some practice in while figuring out the rest of the updated latex world and waiting for orders to arrive. These frames have a bit of a cult following and several here modify them a bit to turn them into very nice shooters. These mods include getting better elastic on them and if this is interesting to you several members here can tell you how to get that done.
Cheers and good luck,