Relay for a 12V light bar
You're planning to use the high beam switch as a momentary contact push button, so you'll want a 12V latching relay of some sort. 

Here is a circuit that allows a single control source to give push on push off function to a DPDT latching relay.  



You have to do your own shopping, but depending on what you find for parts, you may wish to investigate using a small electronic latching circuit to control a non-latching relay.  This would ensure the relay was not left in the energised position when power was cut.  The logic for an electronic latching circuit is more or less the same as for a non electronic one, if you understand boolean logic and some basic electronics you may wish to go that way.  

Seriously consider if you need it integrated with your high beam circuit.  It would be much simpler to control it with an attractive or unobtrusive switch, and using it's own control would allow you to add a dimmer control.

Make sure every component you use is rated for automotive use and the temperatures it will be expected to operate under and the current it is expected to carry.  If you're building your own LED bar, ensure it is well cooled.

You may wish to check out a few other people's homemade bushlight diy videos on youtube.

Local electronics stores may come with helpful staff.  Large reputable online suppliers like digikey and mauser can offer extremely low lead times (next business day shipping) and properly rated reliable parts.  You can also find good parts on ebay, amazon, aliexpress, but always beware counterfeit or misadvertised parts.  Shop around, but try to be very sure of what you are buying.  Bear in mind that most parts have many names and often shopping troubles amount to not knowing what something is called, so bear in mind, a part may have multiple proper names(tissue paper, toilet paper), trade names(Kleenex), make, model, product line name, manufacturer part designator, supplier part designator.  

A MOSFET might get referred to as a transistor(which it is a type of), a switch(which it can be made to function as), a MOSCAP if it's being used as one at the time, and probably a bunch of things I'm not thinking of right now.  Searching for "6A 40V MOSFET" will probably get the best results, but you might have to look around for the magic words.  If you're having trouble finding a part, but you think it should exist, or you're finding fancy, rare, expensive versions of something you thought would be simple, common and cheap, vary your search terms, research synonyms, read a bit on wikipedia.  Be persistent and you should be able to find it.