ILPS 2L-3L Low Profile Hydration + UTA - Product Reviews
The 3L size provides long-term hydration and the UTA allows you to refill from bottles or taps, without having to access the bladder itself. This is great, since you can refill without removing your armor or needing a buddy.
I would definitely recommend this, or a similar bladder in an appropriate form factor.
On a customer service note: There was an issue with shipping that was in no way the fault of Source, shipping company's fault, but Source fixed it immediately and went way above and beyond. Top notch gear from a top notch company.