Depends what you are using it for. Refractometers are excellent for measuring the original gravity of the wort, but a bit dodgy for measuring final gravity as the alcohol content skews the reading and you have to refer to charts.
Hydrometers are good for both purposes, are generally cheaper, but have the disadvantage that you waste a bit of wort and beer, and they have to be cleaned out, dried etc.
Refractometers only use a couple of drops then a quick wipe clean with a damp tissue.
I don't check my FG any more as I can always tell when a brew is finished by squinting into the Sun and checking my corns on the left foot
so refractometer is my choice.
I bought mine from CraftBrewer, other options online I would expect. Also they are in good supply as they are used mainly in the food and agriculture industries, but a proper Beer version is the one I prefer as it has a Gravity Scale as well as
the standard "Brix" scale. Brix does my skull in.
Edit: as you can see Manticle and I focus on opposite ends of the fermentation process - I brew a limited set of styles nowadays so generally know what stage I'm at with each style and only do a reading for OG to see if I've hit target. If you are doing a lot of experimental stuff I'd get both IMHO.