Outside of "legacy dungeons," (big areas with major bosses, like Stormveil Castle) out in the world you will often get some flask charges back when defeating groups of enemies, or when defeating tougher enemies like trolls.

Also, don't be afraid to try melee as a backup if nothing else. Even if you go full INT, there are weapons that innately scale off of INT, and you can apply ashes of war to change many weapons to magic affinity, to make them scale with INT.

Be on the lookout for golden seeds from the small golden trees though to upgrade your amount flask charges.

As mentioned, leveling Mind will help a bunch. I usually play hybrid melee/caster and like to get Mind up to 20-25 for those characters. If you want to rely nearly entirely on magic, you will probably want more (you can invest in it over time, its a big game and you will end up like lvl 120-150+ by the end.)