Classic Balloon Corporation manufactured a nice line of Sandra Boynton mylar balloons.
Sadly, only a few of these balloons are still available.
The Happy Birthday balloon with the cat and confetti and 'Have an Udderly Cool Birthday' are still available from The Balloonery in Stockton, CA.
The cat balloon is here.
The cow balloon is here.
They don't have the cow balloon advertised as a Boynton, so searching for "Boynton" won't find this one.
'Happy Birthday to Yoooo' and 'You're Outstanding in the Field' are still available from Cow Depot.
You can find them here. They are a little way down the page, at the start of the balloon section.
Many thanks to Illona Bobritsky for finding the cow balloons and letting me know about them!
You may note there are gaps in the array. These are placeholders for coming attractions - balloons that I know about but don't have pictures of yet.