I would like names and contact information for wiffleball type/size hard
plastic ball manufacturers in China or Mexico (not soft polyurethane
or soft PVC).

I've found a Taiwanese manufacturer of plastic products, and one of
their sample products appears to be a hard-plastic whiffle-type ball.
They say they also welcome OEM inquiries.

Hseih Chou Plastic Enterprise:
http://customer.manufacture.com.tw/~hsiehchou/index.html

and they list an email contact:
hstracy@seed.net.tw

I also found a website that may be useful. It lists a number of Hong
Kong suppliers for plastic toys.

AsiaTradeLink:
http://www.asiantradelink.com/

In order to get the plastic toys list, you will have to pick "Toys"
from the "Choose a Category" drop-down menu on the left, and then
"Plastic" from the "You may narrow your search" drop-down on the upper
right. Sorry it is not possible to give you a direct link to the list.
From this list, you can check out companies one at a time, or send an
email to all of them at once.

A sample supplier who deals in whiffle-type balls and bats is Grand
Hero Industrial Limited:
http://www.grandhero.com.hk/

They list Mr. Alan Lee as a contact:
info@grandhero.com.hk

Searches:
in Google Directory  Business > Industries > Manufacturing,
"plastic balls sports"

in Google,
"manufacturers listing China"

I hope this helps!

Cheers,
nauster-ga