Is it possible to use reachable as a replacement for available? I noticed that some dictionaries don’t have the former term in their database. Is there any origin for this word?
As a direct replacement, no; “we have 3 types of stew reachable,” is nonsense.
There would be some cases where either could serve, but not as exact equivalents.
There are rewards available to those who work hard.
There are rewards reachable to those who work hard.
Are both close in meaning, though not exactly; both state that the rewards can be obtained, but the latter uses a metaphor of moving toward that reward and getting it, that the former does not.
Here though we’ve changed the imagery of the phrase, so even though they both serve, they still aren’t exactly equivalent. In all, the words shouldn’t be seen as even partial synonyms.