The two I would recommend are the Televue 3x barlow and the Antares 3x Apochromatic barlow (3 lenses like the Ultima). Both offer very sharp and bright high magnification with little loss in image quality. The Televue runs a little over $105 new and $80 used, the Antares is about $75 from a number of online stores including Scope Stuff, Hands on Optics and O'Neill Optical.
Mauricio--You didn't say if your barlow absolutely HAD TO BE 3X. If that number is not critical for you, you might consider the Powermate 2.5X. I've been using a Powermate for a little while now and I'm very happy with the results in my scopes. Nice point for the Powermates: they don't increase the eye relief of the eyepiece you're using, so blackouts don't become a problem. This becomes a consideration when you're pumping up the power on low power eyepieces that already have substantial eye relief. Larry Seguin