My character has the lowest Hit Points in our party, who are all level 10 (I mean he is even lower than the Artificer of the party I am a Fighter)(My CON is 15) so I was wondering if there was any magic items that increase max Hit Points. I am more so looking for one that increases Hit Points permanently. This another question that is similar but different from this one that I asked.
Since you're not maxed-out in Constitution...
Easiest way is by increasing your Constitution.
Items that directly increase HP are harder.
Other non-item options
If feats, epic boons, or multi-classing is an option, then they might be useful for you to pick up if the items are difficult to come by. Multiclassing into Barbarian gives you a d12 hit die, the Tough feat gives you an additional 2HP/level, and Boon of Fortitude gives you an additional 40HP.
As you are only looking for magic items (and not other ways to boost your HP, such as Feats), here are some options. Most of them function by increasing your CON score.
Your Constitution score is 19 while you wear this amulet. It has no effect on you if your Constitution is already 19 or higher without it
You said your CON is presently 15, so this would ultimately be +2 HP per level.
while you are attuned to this weapon, your hit point maximum increases by 1 for each level you have attained.
Warning, cursed item.
If you spend 48 hours over a period of 6 days or fewer studying the book's contents and practicing its guidelines, your Constitution score increases by 2, as does your maximum for that score
Warning: a given book can only be used once a century
Your Constitution score increases by 2, to a maximum of 20.
Your Constitution score increases by 2, to a maximum of 20, while this pink rhomboid orbits your head.
Your Constitution score is 20 while you wear these gloves.
This would, given your current CON score, give you +3 HP per level
I would add that there are also some magic item options out there that generate Temp HP, as well as ones that can heal you--effectively boosting your HP, but you were asking for a more permanent solution, so I left those out.
By increasing your constitution you gain hp per level:
Ioun Stone of Fortitude (very rare) DMG p 176 increases your Con score by 2, so would give you 1 extra hp per level.
Manual of Bodily Health (very rare) DMG p 180 increases your Con score by 2, so would give you 1 extra hp per level.
Belt of Dwarvenkind (very rare) DMG p 155 increases your Con score by 2, so would give you 1 extra hp per level.
Amulet of Health (rare) DMG p 150 sets your Con score to 19. As you have 15 that would be 2 extra hp per level.
Gloves of Soul catching (legendary) Candlekeep Mysteries p 169 sets your Con score to 20. As you have 15, that would be 3 extra hp per level.
Do note that items that set your Con score to a flat value do not stack with other items that increase your con score.
You could use Wish to increase your HP/Con
There are also 5 items in the DMG that allow you to cast Wish and you could wish for an increase in constitution score. Namely: Luck Blade, Ring of Three Wishes, (Eye of Vecna + Hand of Vecna), Deck of Many Things (risky), Efreety Bottle (risky)
Why I think Wish would work: I know at least of one official campaign where a riddle has an ability score increase as a possible reward and wish as a less likely one. So the ability score increase is seen as the less powerful of the rewards. To me that is at least an indication that wish could grant an ability score increase. Specifically here:
ToA area 70, 1d4+1 int increase on a 91-99, wish on a 100
Flat hp increase
Berserkers Axe (rare) DMG p 155 gives you a flat 1 hp per level. But it does come with a downside.
Some items also allow you to cast a spell that can grant you temp hp. But that would not be a permanent increase of health.