With 75 Smithing, use 375 harmonic dust to make one at the crystal singing bowl in the Ithell Clan district of Prifddinas (giving 375 Smithing experience)
The amount of coins needed to create (and repair) is based on the number of times a crystal weapon/armour seed has been made into an item for coins - the cost is shared over all crystal weapons and shields. The first time costs 1,000,000, and each subsequent creation costs 200,000 less until the fifth time (and every time thereafter), which costs 200,000.
The crystal shield will revert to seed after 50,000 charges of combat, but uses two charges per hit except while within Prifddinas, where it uses one charge per hit. A partially degraded shield can be recharged for a fraction of your current creation cost - if it has 90% charges remaining, and the lowest creation price has been achieved, it would cost 20,000 coins to repair.