Losing a wallet is traumatic, but it can be prevented w/ the right training.
Try some of the suggestions below and let me know if you have any ideas, post them in the comments below.
Before I begin, the majority of my clients are Boys. I have no experience with purses. I could come up w/ some ideas but ill stick w/ my experiences to date and what has worked w/ me when helping students stay organized and helping them to manage their money on a daily basis.
Do not keep your wallet in a back back, bag or lunch bag

Wallet Size
I chose one that is small and good for a few cards and some bills, thats it! I also carry my wallet in my front pocket. It is easier to keep track of, easy to remove and less chance of it falling out of my pocket...all of which happens to my students on the Autism Spectrum.

I recommend a credit card wallet. It may seem too small but if you are only carry some i.d, a few bills and a bank card, this is perfect. It also scores BIG points for helping keep your student or child organized.
(I put some links below for smaller sized wallets)
There is no room for clutter in small wallet!
I do not encourage change purses. They are bulky and after a few days downright cumbersome when change, receipts and bus transfers collect w/ in. Change goes in a separate pocket and when you get home, put it straight in your piggy bank.

This is why I want them to get in the practice of having their wallet in the same place ALL THE TIME. It becomes second nature to grab your wallet, find your bills and easy to put the wallet away. This way, the chances of it getting lost or misplaced are reduced dramatically.
Keeping track of your wallet
I also teach them how to keep track that they have their wallet when out. Its a subtle method and prevents the frantic pat down we sometimes do when wondering if we left our wallet someplace, It looks a lot less conspicuous. If you keep your wallet in the front pocket, its easy to find if your hand is resting on your thigh when walking, your thumb can feel the out line of it in your pocket. This way they are reassured that their wallet is on them and they can check as often as they like w/ out the frantic pat down. Same goes for the back pocket, I dont suggest the back pocket as wallets easily slip out of the back pocket or stick out if they are too big (remember my suggestion for a small wallet) and there are pick pockets out in the world looking for an easy target. If your hand is close to your thigh, the pinky finger can feel the outline of the wallet in the back pocket, again, its not as obvious as patting your butt.
I have had a student lose their wallet on the bus. They had on shorts that were extremely baggy and even though they were diligent in checking their wallet every so often, the wallet fell out during a transition. Parents, baggy pants with a light material or shorts are not good for wallets and keys, just saying!
Here is a good article on what to do if you lose your wallet:
I found this article on Wrong Planet about what to do if you lose your wallet. It is written for Older Teens or Adults w/ Autism.
http://www.wrongplanet.net/article445.html
Thanks for reading,
John Anderson
Zippo wallet for men
Much cheaper credit card Holder