Farfisa may have made reed organs, but the Farfisa and Vox organs used in surf bands were electronic, not reed organs.
A harmonium is a reed organ that you play by pumping a bellows-- a "pump organ." Larger models usually have pedals that you pump with your feet, although smaller "tabletop" models can be pumped by hand. If it's handheld then it's probably like an accordion, but having a keyboard like an organ.
Anyway, you won't want a harmonium for playing surf music.
If you want to buy an actual vintage Farfisa or Vox organ, you'll need to buy a used one through Craigslist, eBay, local classified ads, etc.
But it's probably better to get a new keyboard that has some patches/voices/tones* that sound like an old Farfisa, Vox, or similar '60s combo organ. The name of the patch/etc. might not be called "Farfisa Organ" or "Vox Organ" per se-- it might be called "60s Organ," "Combo Organ," "60s Combo Organ," etc.
*Different keyboard manufacturers may call them different things: Yamaha = "voices," Casio = "tones," most everyone else = "patches" or "presets."
A used Farfisa/Vox/etc. is probably going to be heavier than most modern portable electronic keyboards, making it more of a pain to lug around.
And when you buy a used keyboard, you might be buying an instrument that's damaged in some way-- hopefully just cosmetic damages (scratches, etc.), but possibly damage to the electronics, keyboard, or controls.
Plus, even if you get lucky and buy an undamaged instrument, if it ever gets broken you'll need to fix it yourself or get someone else to fix it-- if replacement parts can be found.
And finally, if you should want something that can be hooked up to a computer via a MIDI or USB-MIDI connection, a keyboard from the '60s or '70s isn't going to cut it, since MIDI hadn't been invented then. You might not care about that right now-- and certainly a musical instrument doesn't have to be MIDI-compatible-- but it's something else to consider.
