Sphere of Law is 3W for prevent 2 dmg from each red source -ONLY to YOU-.
Shere of Purity is 3W for prevent 1 dmg from each artifact source again only to you (but was printed in a very artifact heavy set).
This is at least half as good as Sphere of Law, but at 1/4 the price (and in two colors). AND protects much more than just You. How about:

{W/U}
Enchantment
If a red source would deal more than 1 damage, prevent 1 of that damage.