Those spells have a 500xp cost to them and are higher than 3rd level, so potions aren't an option unfortunately.
A say cockring of Fiendish seed 1/day would cost 5040 gold and 25404xp to make, and cost 135080 gold to just buy outright.
Hmm...I thought a single use item wouldn't need the x50 (even if not a potion) but don't see it there specifically. At that cost you may as well just get a ring of wishes (which clearly doesn't use the x50 mechanic!)
Taking that multiple away (so it's just x1 xp, which is logical for a single use spell) gives you:
-Pinata of Fiendish Seed
The user must smash the Pinata while blindfolded to activate the item; this causes the user to produce a half fiendish child when they next copulate, per the Fiendish Seed spell. The Pinata is destroyed when activated.
700 gp + 556xp
Or 4,900 gp market value.
Plus the cost of a masterwork pinata, I suppose.
Or, yeah, Nys could whip up an alteration potion now...or cast polymorph once he gets another level.