Get a headband of intellect
Magical items always require cooperation by your DM who controls access, but you are asking also for magical items that can help.
A headband of intellect will set your intelligence to 19, improving your chances to make that saving throw. It has the added advantages that it is only uncommon, so you could find it or procure it as a quest reward relatively early in the game, and that it is an ongoing effect that works around the clock.
It has a downside in requiring attunement, which at high levels where you may have several items competing for it can be a real cost. As an item it does not use up one of your feats, and you could combine it with a features that give you proficiency on the save like the Resilient feat or like the first level Rogue dip that you took, to nearly be on par with a maximized Intelligence-based caster like a wizard.
What's more, the headband is a generally useful item that will give you a boost to all Intelligence releated ability checks such as Investigation or the various knowledge checks (History, Nature, Arcana).
P.S. If you think about it, it is not a great idea to invest too many resources to defend specifically against only feeblemind. High level spells can all be pretty bad. The odds are it really does not matter that much which 8th level spell is hitting you -- when it happens early in the game, an upcast fireball can destroy you just as well.
Even lower level spells like plane shift (Cha save), flesh to stone (Con save) or disintegrate (Dex save) can remove a character from play, and yet not everyone is trying to shore up all of these saves with their precious feats. You cannot avoid all of them all of the time, so consider tactics that allow you to recover from setbacks, like allies that can restore you.