37mm push on filters and hoods can usually be found cheap to dirt cheap on Fleabay or at your local camera fair. Online it can be a bit difficult to identify the correct size, as most filters haven't marked the size on the rim and the seller sometimes don't bother to mention the size in the article description.
My favorites are the beautifully crafted Voigtländer push on filters and metal hoods in different sizes (29, 32, 37...) which are even available coated (marked "AR" as in "anti-reflex"). They are easy to identify because of the product code stamped on the rim: 301/37 for light yellow 302/37 for dark yellow, 306/37 green, 308/37 orange, 317/37 UV, 325/37 skylight... The 37mm Voigtländer hood 310/37 is a bit of a rarity though. Vintage B+W filters can also be found regularly (at least here in Europe). Agfa also made a lot of 37mm accessories (filters, hoods). This stuff (with the exception of the sought after 310/37) shouldn't cost you more than 5-10$ or so a piece.