Given that all the materials are pretty common, and a working bivy bag can be quite simple in design and construction, little more than a giant stuff sack in some cases, I would encourage you to look into making your own.

Start with a dead simple one, a glorified sack. Take this on a easy trip and modify materials and methods, and add features to fit your needs.

Perhaps start with a simple flat piece of coated nylon pack cloth about 5' wide and 12' long. Stitch on, or arrange buttons to hold a removable, thin microfiber liner over the entire piece. Then fold and sew a 6' long zipper up both sides. Treat the unit, minus liner, with a wash-in durable water repellent. Give it a go and modify or redesign as it takes your fancy.

The cloth is typically available in 59" widths last time I looked but you may want your bag wider or narrower. And the 6' folded length is just a shot in the dark. five foot may be more to your liking. Also, while a simple square bottomed bag is simpler it would be easy to taper the foot-end.

You could also think about using heavier material on the bottom even if this means you can't alternate use top-to-bottom to even out wear. Adding a bit of fleece to the foot under the liner might help. As would adding a pouch near the top on each side for stuff you might need at night.

Of course there two pretty big down sides to this sort of activity: First, it is your baby. If it fails, or fails to please, it is all on you. Up side of that if it works you get all the credit. Making your own gear, particularly when it is better than you could buy, lends a lot of credibility. Any dork with a trust fund can buy their way into good equipment. Those who can make their own are a cut above.

Second, once you start building your own you will never look at store-bought gear the same way. You notice little things like how manufacturers often add pockets in locations that make the item easy to assemble and look right. Usually centered and straight up and down. Which often ends up making the pocket less usable.

You will start to modify your gear. A pack will scarcely make it home before you start taking pieces off.