Getting Latex rubber tubing that is sold for hi-starts is your best approach, but there are others. One option is to buy Theraband or similar exercise tubing. Look at the charts to get an idea of the strength.
Try to find it in full rolls as sometimes they sell it in cut up sections for exercise machine use. I have seen listings for 100' but have learned that it is actually four 25 foot sections You can join the sections but it is better to have a solid roll.
As you know the rule is that the hi-start should have at least 3X the pull as compared to the weight of the glider when you pull it 3X its rested length. Remember you don't have to pull it all the way to 3X length to launch. The chart below shows the pull with a 1X and 2X stretch. We are going to go up to 3X so multiply accordingly. Looks like a 1.5 factor from 200 to 300%.
For under 2M gliders or very light 2M gliders in the 1-2 pounds range, the red should work well.
If you are flying 2M to 2.5 meter gliders, in the 1.5 to 3 pounds, the green should be good.
2M to 3M gliders in the 2 to 4.0 pounds the blue should work.
If you gliders are typically over 3 pounds to about 4.5 pounds the Black is for you.
If your glider has a fairly strong wing you can go up a color for a stronger launch. I have launch my 35 ounce Spirit at 12 pounds of pull and I launched 28 ounce Gentle Ladies at 10 pounds of pull, around 6X the weight of the glider but it is not necessary to go that high.
And if you have some heavier glides or want a STRONG pull for a STRONG launch on a strong glider then get the silver or even the gold!
My hi-start would be between black and silver. I have launched 2M through 4M planes from 30 ounces to 85 ounces from that hi-start. On the 2M I may only pull it back 2X. On the 4M or heavier planes planes I pull it all the way.
How much pull can you handle?
That is a personal decision. I can handle the full 24 pound pull on my hi-start by myself but it is a bit tricky pulling, holding and releasing the plane with one hand while holding the radio in the other. For many people anything over about 16 pounds of pull starts to become hard to manage alone. By my standards 16 pounds of pull should be good for a 64 ounce glider in calm air and up to about a 75 ounce glider with a 5 mph breeze.
Amazon has all the colors in 100' lengths but they are actually made up of 4 25 foot sectons that you would need to join. Joining is not hard but a solid piece would be better.
here is a RED listing
http://www.amazon.com/Thera-Band-Res...eraband+100%27