Start writing letters and bids. Your local Rotary Club, Lions Club, City/Town/Parish/County/UA Council, RFCA, Masons, big businesses with community/social/corporate responsibility programs can all throw money at you. We’ve found writing to them saying “we’re looking for match funding” or specific items gains a better response than “please buy everything”.
A charity I worked at previously got gifted 5 x Sony 28" flatscreens - with HDMI etc - all because they were upgrading their tradestands and would’ve had to pay for disposal of them! They were less than 12 months old. They also paid for extended warranties for us to “top up” the donation.
A little bit of Facebook marketing as a thank you always goes someway to helping afterwards!
Our flight sim came in at about £1500 - it depends very much on what you want to achieve with it, how much space you have and 101 other things. Ours, for example, we have a shared building with the ACF so, unless we want to clean crayon off the screens every parade night, we need to lock it away every night. That limits it’s size/space/weight requirements - as such, ours is built inside a bespoke mobile cabinet. Which meant tapping up Jewsons & Buildbase for offcuts and donations as well as the Tech companies!