However there is going to be a Green Arrow pilot, and then possibly a series. Though not a spin-off of Smallville the project, aptly named "Arrow", seems promising. Also in pre-production: - A Booster Gold Pilot (presumably unrelated to Smallville)
I can advise you to check out "Birds of Prey", a short-lived TV-series that is almost a spin-off of Smallville; at least it shares some producers and creative license, if you haven't already seen it. It is available on DVD. Alfred Gough and Miles Millar (the creators of "Smallville") have also produced an "Aquaman" pilot, starring Justin Hartley (before he played Green Arrow on "Smallville".) It is available in the "Smallville: The Complete Series" DVD box set or on iTunes.