If you have a LOWES around where you live you can as jmp said the exact measurement its labeled as latex hose with a 3/8" OD and 1/4" ID this tubing works very well with heavy ammunition. Get yourself a good pouch that can accomodate 1/2" hexnuts and stones cos you never when you will need them
Also this tubing works well with heavier ammo like the 3/4oz lead egg sinkers *OUCH* that smarts
That will stop most any prey you shoot with it
My father swears by this stuff.. Personally I use chained bands but I have had my experiences with heavy tubes like these and they absolutely get the job done without struggle. The plus being great longevity..
Let us know how it turns out and do post kill pics
Note: I asked the clerk at LOWES if they had surgical tubing and she directed me to plumbing where they had hoses and thats where I found the latex hose which I feel will give you the most satisfaction in your tube style hunting slingshot.