You just have to get lucky and find it as a drop or on sale as a special. The hellbug Arkfalls with the crystals seem to drop a lot of shields.
With shields you have to look at three things:
Max shield
How long it takes to start recharging
How long it takes to charge up
I think ironclad takes 9 seconds to start recharging? That means you have to be behind cover for 9 seconds without being hit for 9 seconds. That is often very hard to do. Resparks have the lowest max shield value, but start recharging in 2 seconds. So you just pop behind cover for 2 seconds, and you recharge very quickly.

