It depends on which game you are playing.
HM 05 is flash
In Pokemon Platinum, Flash is a TM.
Look at "How do you get hm flash in blue?" On this site.
on Pokemon yellow on Pokemon yellow
its not an HM anymore you have to find it.
You can get the Hm flash of Pokemon silver is from the elder on the top of sprout tower in Violet city. There are alot of trainers through out sprout tower
you get the HM Flash after beating the electric gym leader, if i recall correctly
flash isn't a HM but a TM and you can purchase it at goldenrod city super store thing
you get it from a gy
There is no HM Flash on Pokemon Diamond, I personally think its been replaced by HM05 Defog.
it should tell you when your teaching a Pokemon a TM/hm!
there are many MANY Pokemon that can use the hm flash in Pokemon sapphire. the following Pokemon can: grovile( treeko),kirlia(ralts)shroomish, eletrike, Roselia, plusle, munun, magnemite, voltorb and many more