I recently got this changing bag on Amazon for $18 and am very happy with it.
Although very inexpensive, it has double cuff seals and a single zipper with a secondary velcro cover that works well. My 30 year old loose cuffed rubber lined bag that I used only in darkened rooms and was becoming gritty on the inside with black rubber grit. Get the larger sized bag, it's very roomy and has a smooth shiny inside liner that will not moult rubber. It is very nearly air tight, and you can feel this after getting your hands in there when you try to raise the roof.
I use this bag indoors only since it has zero ventilation. Be careful with the velcros that are stitched on and it should last at least a few years. I was afraid to get a new bag for some time but very happy with this one. I remember paying about twice as much for my 30 year old rubber lined monster with only single cuffs, and this bag is less than half the weight but is slightly larger insidde. Simple is a good thing.