It depends on what you mean by "transfer switch". Your existing electrical panel is already a transfer switch, what you need is an interlock kit that makes it physically impossible to use the mains and the generator at the same time. This isn't the exact model that we have but it shows you what I'm talking about (the type you need is specific to the type of electrical panel you have):
https://www.amazon.com/SD-200VL-Generator-Interlock-Vertical-Breaker/dp/B07S3JHYMV/ref=as_li_ss_tl?dchild=1&keywords=generator+lockout+switch&qid=1598505416&sr=8-6&linkCode=ll1&tag=pw016-20&linkId=06cce8e6ba43fe4dfa498b95c6b23a63&language=en_US
In addition to the lockout switch you'll need the receptacle installed that accepts the power from the generator, the cord that goes from the generator to the receptacle (HD sells them but Royall made ours for cheaper), and of course have the receptacle wired to the panel.
The above is the cheapest solution other than an illegal "suicide plug" which just routes the power from your generator to your stove or dryer receptacle and counts on you remembering 100% of the time to kill your main breaker before adding the generator and also not to touch the live prongs on the modified generator cord and killing yourself. (not recommended).
For several thousand dollars you can upgrade to an automatic transfer switch that not only switches back and forth between mains and generator when the grid goes out but even starts the generator (if the genny supports that function). But I doubt many people in Puna have something like that.
Generac and other companies sell systems that are essentially a second electrical panel- you switch between the mains or generator panel which is what I originally purchased. They are more expensive and give you less capabilities than the cheap option I went with, but they might be easier to operate. This is the one I sent back:
https://www.amazon.com/gp/product/B007SR6HQ4/ref=as_li_ss_tl?ie=UTF8&psc=1&linkCode=ll1&tag=pw016-20&linkId=e3ca60255bfb80159ab2ff6a207e5b62&language=en_US
Basically the problem with these is that it's not really a "whole house" solution because you can only run the legs on the mini panel with the generator. I guess it's purpose is to make sure you can't overwhelm the capability of your generator, but some of us are smart enough to know you can't run your whole house including electric dryer, hot water, and oven/stove all at the same time without overwhelming the generator. You can either just remember not to run everything at the same time, or for example, not turn on the switch for the dryer and hot water heater on your regular panel when the generator is running.
In other words, you already have a clumsy transfer switch in your house, you just need to connect it to your generator and make sure the grid and generator don't add power at the same time by paying $70 (or whatever) for a $5 piece of metal (UL listing isn't free!).
https://www.youtube.com/watch?v=n7DkaorEQPQ