I want to buy an item that is not popular and not on the store.

I want to buy an item that is not popular and not on the store.
It is a low lvl quest reward. I have finished this quest during leveling... and just like with all low lvl quest rewards I have probably sold or not picked it.

Can an admin contact me and help me with this, please?

They can't help you with this. There's no way for you to get that item anymore.

If I were you, I'd look up items with identical models, most items in the game have their models recycled so there's a good chance you can get the item still if it's for xmog purposes.