Check out the marine supply stores. I have found 'boat bags' in the past that work well and were water proof or very water resistant. Some guys just use dry bags for pannier liners too. They can be found in all shapes and sizes. Right now I am using a very nice BMW pannier liner that is waterproof and zips up with handles. I stole it from my wife.
(she only uses her top case liner on her R1200GSA) Point is, I bet you can find all sorts of pannier liners on ebay for other bikes that would fit fine in your Givi Trekkers. And maybe for a good price if you hunt around. I bought an aftermarket liner bag off ebay for my previous Touratech pannier that was quite nice, if not as heavy duty as the BMW one I use now. IIRC, it was only around $30. I still have that, it was for a 38L pannier. Rubberized, side opening zipper. Not sure if I'd call it waterproof, but very water resistant. The pannier is waterproof, eh? PM if you want pics.
I guess your needs will dictate what you want. I use one pannier for tools and spares, etc and do not use a liner. The other side is clothes and I use the liner there so I can pull it out to take in to the hotel room or tent easily.