need ideas for adding holder for water bottle in sling bag

You might look at purse organizer inserts to see if anything would fit.

Take a look at this bottle sling from ChicoBags:

https://www.chicobag.com/collections/travel-bags/products/bottle-sling-repete?variant=29509491556467

You might be able to attach it to the inside of your purse with the carabiner. Or else you can sling it over your other shoulder when you go out?

It's really lightweight and thin, and packs up small. I also use a ChicoBag tote for my sweater and other extras.

Have fun!!

I use a Vapur collapsible water bottle (find on Amazon) for business and vacation travel. Has an integrated clip that I use whether it's full or empty, inside and out of my Osprey pack. Just clip it and go.

I use this type of bottle clip that attaches to the neck of the bottle with a two-pronged clip and to your bag with a carabiner.

i used to use a bottle holder with strap from Build NY which doesn't seem to be available currently, but it was something like this:
https://www.amazon.com/Neoprene-Carrier-Children-Cycling-Climbing/dp/B01F3YZNRW/ref=mp_s_a_1_4?dchild=1&keywords=neoprene+bottle+holder&qid=1581615969&sprefix=neoprene+bottle+holder&sr=8-4

The Chico bottle sling is great! I have used it while travelling and it works really well....lightweight but strong, fits a variety of water bottles and I even use the pocket to put my cell phone in. And after a trip, it washes up nicely on the gentle cycle in the washing machine.

Thank you everyone for your replies.
The bottle sleeve would be better than the ziplock I usually stick it in.
Will have to look up purse organizers, and perhaps there are backpack organizers as well

I use a Platypus water bottle that collapses down, I carry it in my sling bag, much easier than a regular plastic water bottle. It’s also nice for going through TSA and then filling gate-side at the airport.

If you have a TJ Maxx or Marshall's nearby and it's a regular stop for you, lol, watch for purse inserts there. I found one at my TJ Maxx last year on the mark down rack near the cash registers for about $5. I've used it in both a sack type purse and on my last trip used it as an insert in my Tote bag. It worked great for both and it does have enough room to accommodate a water bottle across one end.

If you look online you'll find lots of cheap ones at Amazon but there are some really cute (and MORE expensive) ones on Etsy. Look for purse insert or organizer. I'm currently looking at cute ones with Paris themed fabric....sigh....

When I needed a large external pocket on the back of my PacSafe purse for stuff like chapstick and kleenex, I went to a fabric store and bought some really, really wide black elastic (maybe 4" wide?) that is used for making elastic waistbands. Sewed two pockets (using just the length of elastic, no additional fabric) that did a great job of holding stuff in without needing to be fastened in any way. Perhaps you or somebody you know could make a long, elasticized pocket for your water bottle on the inside or outside of your sling bag.

More good ideas!
I do sew but have gotten lazy. I had considered sewing a pocket on side of bag or just sewing a divider into one of the large pockets. But fears of botching it up on a bag I really like quelled that urge.
I did look on line yesterday but would actually prefer seeing something in person. I do have a TJ Max and a Ross nearby I think I will check out what they may have.
Thank you for suggestions.

Here's an update on adding holder for water bottle.
I put a strip of Velcro in the large pocket inside to create a separation to contain the water bottle. The type I had in my sewing box was the self sticking type so we will see if it will hold.

I have a bottle holder like the Chico one listed above. Got it a month ago at REI in Berkeley.