Correct spelling, explanation: this is a correct form, however, it is more often spelled in-store. This adjective describes a situation when something is available, for example in a shop. It is a merge of words in and store. Instored is incorrect, but there are sources that describe instore as a verb (with similar meaning as to store), which would make instored an actual, existing past simple/past participle form. We would take this with a grain of salt because none of the bigger dictionaries suggest this meaning.

Definition of instore (in-store):
adjective, something in stock of a shop or magazine, accessible
I’m sorry but we don’t have this sofa in-store anymore.
I wanted to buy this book, but it is nowhere instore anymore.

Incorrect spelling, explanation: there is no such word as instored since in-store (sometimes: instore) is the correct form. Instore is a combination of two words (that is why it is often written with a dash): in and store. However, some sources suggest that instore is also a verb, which would mean that there is such word as instored (past simple/past participle form of a verb). Possibly this is mistaken with a verb to store.