recommendations for finding aircraft
 

Any suggestions for the US/UK/Europe on the best way (online sites, publications) to connect with owners selling aircraft, preferably avoiding brokers and their markups.

I think most owners use brokers so they don't have to deal directly with skid-kickers, dreamers and wannabees. Commissions are not that onerous, generally.

Probably the nearest thing to what you are after is eBay (seriously!) or Helidata in the UK, which usually has a couple of direct sale ads.

You probably also need to be more specific about what you are looking for - model, budget, location...…
There will be people who have, or know of, helicopters that aren't being advertised but might be for sale.

If talking helicopters and in the US, it's more a niche market. So depending on type most helicopters are sold through brokers or dealers. However, there can be the odd deal out there. Post in or review Trade-A-Plane, Barnstormers, and Controller for your best chance.