You can pick up the 90 degree valve at REI, BassPro, or Dick's Sporting Goods. Seal it to the end of the tube with a little Goop so it won't slide off and dump your water. The bite valves are good until they start leaking then it's a good time to put the 90 on.
I use a vinyl coated MSR bladder with this set up when I backpack and it's tough. Also, if you get the bladder with the same size mouth as the Nalgene bottles, they will screw onto a water filter and you can reach into them with a bottle brush to scrub. Rinse with a little chlorox solution (dilute pretty good with water) after use and let them air dry with the valve open and you will have better tasting water the next time you use it.