The nylon/elastic purse holsters that most manufacturers include with the bags are a place to start, but they will not be a good fit for many guns.
Your gun won't "snap in" - these holsters have zero retention. If you tip the holster upside down, the gun will slip out. We strongly recommend getting a kydex purse holster that's molded for your specific gun model, with snap-in retention.
You'll get several loose adhesive velcro sheets with it, so you can attach it however you prefer inside your bag (or any surface).