I borrowed some of the cost of an aircraft for my company a few years ago.
My bank offered an overdraft. Too expensive. Although I dealt with finance companies on a daily basis I could not get any of the big names to lend for an aircraft. One said they had been left with five Boeing 737s some time ago and did not now get involved with aviation. I eventually used Air and General finance, now Close Brothers. They insisted on a mortgage although the loan was only 5 years.
I think the cheapest way now for a private buyer would be to take out a personal loan and say it was for a house extention.
DO.