Can I use a buffer for a "To Mixer/PA" output for bass?

[GrooveChampion]

And if so, which do you recommend?

[Neil mcNasty]

NO! It will not sound very good!
A buffer is only affecting the impedance and always stays at unity level.
So the signal stays the same, but is more suitable for long cable runs (when you use a buffer).

A DI box is used for this purpose.
Without it, you'll have to turn the gain way up, and therefore you will get a lot of noise and hiss with the signal, since the bass signal is very weak compared to  the line level that a mixer is designed for.
An active bass has a stronger signal, but still not as strong as line level.
You should get/build yourself a DI box!

Sometimes there is a DI out on bass amps that is used to send the clean bass signal to the PA/Mixer, but I guess you are talking about situations where you do not have such a bass amp available...

[GrooveChampion]

Then is there a vero for a 9V DI box?

[Neil mcNasty]

There are not any on this site, as far as I know...
Except for the Red Box that runs on 12v. But if you build this one make sure that you read the instructions properly: http://tagboardeffects.blogspot.no/2016/10/hughes-kettner-red-box-classic-di.html

Normally D.I.Boxes are run on 48v Phantom Power fed from the mixer, and a good D.I comes with a high quality transformer inside, so the cost of building one yourself is often higher than just buying one in the shop (which I highly recommend! Check out the ones from Radial, those are the best!)
Parasite Studio has a quite simple transformer-less D.I.Box build that runs on phantom power from the mixer, and can be found here: http://www.parasitstudio.se/stripboard-layouts/active-direct-injection-box-di

Then there is this one from TH-Custom, that uses 9v and a charge pump to raise the voltage: http://diy.thcustom.com/shop/balanced-line-driver-v1-0/

Have you seen this wonderful unit built for bass players?
I'm going to get one of these soon: http://diy.thcustom.com/shop/all-in-one-v1-3-pcb/

Hope this helps pointing you in the right direction...

[Neil mcNasty]

Here's a Vero of the TH-Customs "Balanced Line Driver" that I drew up last night...