You can hse your own blood to do spells

*use

It will be drained out of your body when you cast spells, although because you need at least 40-50% of blood to survive, you won't be able to go below 50%, so really you only have 3l of blood bank. You'll be heavily weakened after using spells so you have to keep that in mind. The body can produce about 0.5-1l of blood per day, but it can vary on several factors like the amount of iron in your diet, your hydration and if you have hemochromatosis disease, you can produce a lot more blood than an usual person, so technically you could even make 3-4l per day with it if you heavily optimize it, just keep an iron rich diet.

The spell cost will depend on its power, so for example creating sparks will cost only a few drops, levitating might drain like 30-50ml per second and a fireball will be able to drain your entire blood bank

You can also fabricate things, although it cannot contain iron (so you can't give yourself more blood), the summoning cost will be linked to the stock market value of the given resources (10ml of blood = $1)

Although there's also a bonus levelling mechanic: spell cost will be multiplied by 0.95^{sqrt(bloodUsed),} which essentially means the more blood you use, the cheaper your spells will be (for clarity, blood is measured in liters in this formula), so -5% once you use 1l, you get -5% spell cost, 100l is -40.2% spell cost and 5000l is -97.4%, it's just that you have to regenerate that blood and the fact that if you use up too much of it, you'll feel very weak.