I did my first pants filling today and loved it. I'm looking to get plastic pants for next time. Does anyone have any recommendations? I can only find sellers for incontinence and was wondering if someone sells pants made for WAM. I also see pull-up and button snap pants. I think pull-up would be better but they say both are watertight.
I'd appreciate any advice you have for this.

Not specifically for WAM but pvc-u-like do a wide range of plastic clothing for fetish purposes, including plastic pants: https://pvc-u-like.com/Pull-on-Pants__c-p-0-0-57.aspx

Those plastic incontinence pants are probably your best option;
They're waterproof
They've got elasticated legs, design to keep stuff in
They're baggy, with plenty of room to hold stuff.
And they're cheap, with a choice of suppliers who offer discrete packaging.

Pretty much perfect for messy play and filling with various substances.
The only downside is they don't look great.

If you've got a bit of extra money to throw about, head on over to the AB (adult baby) fetish suppliers. They offer a wide selection of detailed and great looking PVC knickers and nappy covers.

PVC-U-LIKE also has a great eBay outlet too, you can get things a little cheaper since its been worn for photoshoots.