Child's rain poncho, $2.11 on Amazon. Adult rain poncho, $1.49 on Amazon.

Amazon has got this child's rain poncho for $2.11.  Its 30" x 40.

Amazon has also got this universal rain poncho for adults in navy for $1.49. Its 50" x 80".

We keep ponchos like this in our cars' glove boxes for sudden bad weather. No quantity limit on either item.

These are both Amazon Add-On items, which means that each ships for free with a $25 minimum order if you are an Amazon Prime member (and the price of each item counts towards that $25), or each ships for free with a $49 minimum order if you're not a Prime member.  You can buy multiples of any Add-On item to help get you to your $25 or $49 minimum for free shipping.  (Other Amazon Add-On item deals can be found here.)