I personally prefer the 9012 HIRs for the 9006, and the 9011s for the 9005s, as an upgrade.
Yes, you should replace in pairs since they're both the same age, and if you don't swap in an OEM -exact replacement, you risk having a really funky looking light path.
THAT said - for purchase in the local stores, you have to weigh increased brightness against longer life (i.e., the Silverstar Ultras that have such bright lights? Last about 1/4 as long as the OEM grade lights ... )
As to the $29.95 DDM kits - you can try those. My experience is that, without the proper projector housings for them, you end up with a bright flare of light at the car (so you think it's brighter), which causes the pupils to contract (reducing night sensitivity), and ... there's not that much light further down the road. I'd recommend against them.
For the HIRs, you can find them on Amazon and Ebay. There's a write up on them
here, but you don't have to BUY from there.
RwP