Gari as subtitutes for tapioca
Most western substitutes call for using Corn Starch instead since the two have similar binding properties. For example see this:

http://www.buzzle.com/articles/tapioca-flour-substitute.html

Garri/Gari may seem closer to home, since it made from Tapioca as well (aka Casava), but it is a different composition than Tapioca flour. While Tapioca flour is primarily starch, Garri's composition is somewhat the opposite since Garri is what remains after the starchy water is drained from mashed-up, oiled Tapioca.

As suggested, look for starch-based flours, your best option is Corn Starch available in most countries.

Sources for my research:

On Tapioca Starch:
http://www.bobsredmill.com/blog/healthy-living/what-is-it-wednesday-tapioca-flourstarch/

On Garri: Wikipedia :)