Just about any bike shop or bike coop will have a bin of discarded inner tubes. Most places will be glad to give these away just to get rid of them. I like the long thin ones (27" racing tubes) for most ties I make. Making one takes about 10 seconds so you can keep some readymade or make them as you need them.
Step 1: Cut Inner Tubes to Length
I find 18" and 24" lengths are pretty handy, but the nice thing is you can make these pull ties any length you need.