How much are you willing to spend, how many tools do you have on hand, and how much work do you want to put in?
If you go to the Mandalorian Supplies thread in the Trading Station you can find multiple people selling anything from ABS vacuum formed bucket kits to fully finished resin cast helmets. Or you can get one of the Rubies Jengo two-piece or the Black Series Boba Fett Helmet and modify it. Or, if you're really locked into a small budget, a hard hat or bike helmet and hardware store buckets. If you're feeling a bit extra there are also pepakura files here on the forums and go with a papercraft and fiberglass.
https://mandalorianmercs.org/forum/index.php?board=277.0Obviously a fully finished resin cast helmet will cost you significantly more than an unfinished bucket. It literally is only determined by exactly how much work and money you want to put in at the end, the less finished the product the more work; pepakura and fiberglass with create the most work, next is the helmet/bucket, then the various kits/toy mods. The helmet/bucket DIY option is one of the cheapest with "Kick the Bucket" (
https://mandalorianmercs.org/forum/index.php?topic=170297.0) being the most affordable bucket sold on the Trading Station. Pepakura and fiberglass is about equivalent in price to a resin cast at about $160+(US).