NEHerp and Black Jungle - both - have great moss for sale.
Folius has moss for dusting.

We all do things a little differently
Personally, I would NOT use any moss from outside in any of my frog enclosures.

I have used both suppliers mentioned above. They are super!
When I purchase from either of these 2 suppliers ; I don't use it immediately.
I store it in gallon zip-lock bags under the plant light--- for at least a month.

No disrespect to plant and moss suppliers:
Imagine this.
You build a new PDF/frog enclosure ; using plants and moss that state "ready to us",
or someone ( not you ) says they are safe and ready to go.
This might include native mosses that have been so-called "de-contaminated"
You let the enclosure cycle while your frogs kept, diligently, in QT for a minimum of 30 days.
That exciting day comes when you are going to add your $$$$ PDFs to their new home.
6 weeks later the poor babies are sick, failing to thrive ?, ie and eye issue or white fungal patches on their skin !
Now what?

It's all about the frogs