My players hit with a few questions recently, and since I archive my answers online, here they are.
Can I get higher-skill potions, so they're more likely to overcome Magic Resistance? - No. In my game at least, potions are not bottled spells, but magical concoctions. No matter who makes it, they get an effective skill of 15 once it's created.
I agree higher skill potions make sense, but they add another stat to track, need a cost structure to price them fairly, etc.
Can I get higher-skill Spellstones? - Yes. Magic items assume the maker takes on enough helpers to speed up the process without dropping below skill 15, and thus they all have skill 15 unless you pay extra.
Cost is generally 3x as much for skill 20, and +1x for each skill point past that, based on GURPS Magic.
But importantly you need a caster to work alone, instead of with a circle, and generate a lot more mana without using any helpers. Or you need a lower-skill caster group using a lot of extra power.
So the price in my game will be for Slow & Sure enchantment, $20/point, but then multiplied by the skill multiplier. Skill 20 would be 60/point, Skill 21 80/point, etc. So even a 1-point powerstone would cost 20 to enchant x 80 = 1600 + 70 for the stone or $1670.
Expensive, yes, but it's a special-order type of item in very low demand and which takes so much extra time to create.
Can I get higher-skill magic items? - Yes, but don't bother. It generally won't do anything for you, since I so rarely use Low Mana Zones, and magic armor/weapons don't stop working in LMZs in this game.