I haven`t seen these mentioned on here, so thought I would make a post.
The common substitute for IAL around here is not oak leaves or rooibos tea (recently found out that the tea is actually discouraged by Japanese breeders because it only gives color - rooibos is colored by natural pigments and not by tannins. I guess it has incredibly low tannin levels.)
The preferred substitute are Alder tree seed pods.
The ones that look like little pine cones.
Like these; (Picture not mine)
Dry them out, shake the seeds out, and you can make really great water. These may be easier to get than oak leaves for some people.