The Brompton handlebars are not custom-sized. You can use just about any non-oversized MTB bar that has the 25.4mm stem clamp. I mocked up a M-stem with an old Bontrager Ti handlebar and PDR is correct in that the cable housings needed trimming. However, that's not hard if you're handy around the bike. The handlebar may also need to trimmed for length, so that it doesn't affect the fold.