Some communities have a bike coop or a bike charity that accepts used bikes that they fix up for discounted sales to the poor. Often those places have bins of used parts for sale at a good price. I have bought used handlebars, stems, brake levers, etc. at such places.

Drop bars - when your hands are on the brake hoods, your hands are quite a bit foreward of the steerer tube (the tube that connects your stem and fork together is the steerer tube). But your hand positions on flat bars are usually much closer to the saddle than on drop bars. Therefore, if you make the change you might find that you have to lean much further forward than you had planned on. A really short stem can fix some of that, handlebars that have a short reach can fix some of it too. But you might not find it to work out the way you want.

A friend of mine used to use drop bars, but he never used the drop position on the drop bars, thus that part of the handlebar was useless to him. He set up his touring bike with bar end shifters on bull horn bars, I attached a photo. They ride very much like riding on the hoods of the brake levers on drop bars. I prefer drop bars but in strong headwinds or fast downhills I use the drops to have a more aero position.

The butterfly type bars that you are also considering would likely mean that the bike would ride the same with the same forward reach. Depending on how you configure the bars, you might need a longer stem too.