Here's some information you probably don't need (yet). I use a Kokatat-branded hydration bladder that is actually made by Hydrapak. Here is some information about part replacement and maintenance. The slider closure of the bladder can become very hard to operate, but a little bit of food grade silicon lubricant works wonders. The drinking tube and mouthpiece can be replaced with hydrapak parts or parts from other manufacturers of hydration systems. A cleaning kit for the hose can be obtained from hydrapak; the hose tends to get populated by an unspeakable micro-biota after a while. I had to replace the o-ring in the quick disconnect (hose to bladder connection); the following size worked: 8 mm inner diameter (ID) by 2 mm cross-section (CS) by 12 mm outer diameter (OD). In general, o-ring dimensions can be specified by any two of ID, CS, and OD. Lubricate the o-ring with a tiny bit of food grade silicon lubricant, not cooking oil, etc.
Edit: Don't believe everything you read on the internet. The o-ring leaked. The only solution I found is to buy a new quick-disconnect from hydrapak. $7 part to replace a 50 cent o-ring.