Can anyone tell me the correct size the hole diameter inside the hose is?
I don’t think a definitive answer has ever been found.
A lot of people seem to go with a 1/2" outer diameter hose, but a 1/2 inch looks a little too thin to my eye.
I went with a 5/8" OD hose with a 3/8" ID for a nice rich, screen accurate (to my eye) yellow.
I got mine from mcmaster.com (instructions on where to find the hose on the site it in a minute).
There’s a ten foot minimum order at a $1.38 a foot (for a 3/8" ID). My only issue with the site is that they will not tell you how much shipping costs (you won’t know the actual sale total until the item and receipt are physically in your hand). For me (in Ohio) the shipping was only about $4.12 though.
They actually offer clear yellow 5/8" hose with three different Inner Diameters (3/8", 7/16", and 1/2").
Of the three, the 3/8" ID is the heaviest (it has a noticeable weight to it), the deepest yellow, and the more expensive option. If you have a soft hose connector and/or are worried about wear and tear on your costume you may not want this one.
1/2" ID is the lightest (it’s surprising light), the palest yellow of the three (but still a decent shade), the cheapest, and softest (it’s very easy to pinch the 1/2" ID hose, unlike the 3/8" ID which is very firm yet still very flexible). You’ll probably want the 1/2" if your costume and hose connector is on the fragile side.
The 7/16" ID his a happy middle ground on all fronts. Medium weight, medium yellow, medium firmness, medium price.
I’ll try to post a photo of the three side-by-side shortly.
Anyway, to find the hose you want, first click this link…
Scroll down to where it lists colors, and under “Clear” choose “Yellow”. From there select an OD of 5/8", the page will refresh, and then pick whichever inner diameter you want. 
Ok, I don’t think either of these photos does the color and clarity of these hoses justice, but it was the best I could do in low light and short time.
The first was taken in incandescent light with a flash (and I still had to bump the brightness in Photoshop). There’s a very very small smidge of orange tint to the hoses IRL that gives them a nice rich yellow, but this photo is misleading and makes the orange appear a lot darker and more obvious than it is in real life…
The second was taken with my scanner. I still had to bump the brightness and tweak the hue a little bit to get them closer to their actual color, but again, I’m working in incandescent light so I had to wing it.
Also feel free to grab whatever hose diameter you feel is correct in clear and use “Golden Yellow” RIT clothes dye and boil it up on the stove and dye it to the color/ shade you feel is right.
It ain’t working for me. Everytime I try to get an ID of 3/8 it keeps giving 9/16 for OD
The McMaster site has changed a bit since I made that post.
The 5/8 x 3/8 hose is item # 5187K73 McMaster-Carr
The 5/8 x 1/2 hose is item # 5187K75 McMaster-Carr
The 5/8 x 7/16 hose is item # 5187K66 McMaster-Carr
How long did it take for them to ship the hoses?
Sorry, I don’t remember. It’s been a few years since I’ve ordered anything from them.
the links don’t work anymore, and it doesn’t tell me if the color I’m buying is yellow or not
I’ve had luck with 10mm I.D.
Never mind for some reason the links work on my phone and on my computer weird
Thanks for this tip i bought some 1/2 inch tubing to fill out an amazon free shipping order and was wondering how I was going to make it yellow.