We've had a bit of a problem with the zip feature of Zip-locs [or Aloksaks or OP Saks] getting filled with whatever powdered drink we put into the bags. What alternatives to these type of bags do others use?

Many thanks.

Simple. Don't put stuff in the zipper. Open it up and fold it over twice so nothing gets in it. Problem solved.
You could also use Nalgene canteens. They have a wide mouth and flexible body, but I manage with careful pouring into a .5L Platypus or water bottle.

I use platypus bottles for a lot of powder or 500ml soda/water bottles for smaller amounts. This system has proven bomber for me when having essentially all my nutrition in powder form. Light water bottles with secure 28mm caps, not 26.7mm water caps, can weigh as little as 15g for 500ml and they don't explode in your pack.

Use a plastic bag, NOT a ziploc.

Just tie it in a knot, it won't come undone. And then, ROLL the top of the bag down before pouring. You can pour with exacting accuracy, and no mess.

It weighs LESS than the platypus, and it's free. You must have a drawer of old bags in the kitchen - right?

I just don't trust plastic bags. Stretch blown PET bottles, even the lightest water bottles, are SO much stronger, abrasion and puncture resistant. Plus pouring out of a bottle is even easier than a bag.

For those of you who carry Perpetuem/Heed/etc, just try the bottle method. You won't worry about your powder exploding in your pack ever again.