For my two piece carbon 220 cm Greenland paddle, I snagged a pink camo padded rifle case for $11 at a discount store. Works perfectly but probably would not fit most standard blade paddles.
You may might find cheaper options in bags for other sports and also at music stores. I have a padded “gig bag” for an electric bass guitar that was under $40 and would fit most paddles. Has to be for a bass guitar because a standard guitar case is only 40" (so it would only fit up to a 205 cm paddle). A bass guitar case is usually 47" which should fit up to a 230 cm 2-piece paddle.
For my one piece wood Greenland paddle I sewed a polar fleece “sock” for it.