Holga is now selling lenses for DSLR. The Nikon version HL-N is a 60mm f/8 all plastic lens that costs $24.99 and weights 38g. Other Holga lenses and accessories are compatible with the HL-N (Lomography has a similar solution). The lens is also available on eBay.
In my Lensbaby hands-on report from few weeks ago, I mentioned that I did not like the lack of a built-in aperture ring in the Composer. Two days ago Lensbaby announced the new Sweet 35 Optic that features a 12-blade adjustable aperture. The lens is compatible with the existing Composer, Muse, Control Freak and Scout lens bodies.