Yes I was very close to the border (Peru/Brasil/Colombia) in July 2012. Theres a village on the peruvian side called Santa Rosa or something. Theres also a Customs office in a all three countries. Theres villages on the amazon river from all three countries as well. The colombian one is leticia but there might be a smaller one next to it, as I only went from Iquitos towards Manaus. They do drop you off in colombia on the way so they would do the same from the other direction I suppose. There are however a few issues with the customs personel... lets just say they are only there if somebody asks them to be there... a few days in advance AT LEAST. Captain of my boat offered to call, and said if you dont you might end up having to wait several days there until somebody comes. Immigration is no problem, just the customs and bike paperwork. Another thing is, I dont know how the roads are from Leticia further into colombia. Its one of those towns saying you can only take a flight, but it says the same about Iquitos in Peru, and I know that there are roads to Iquitos from Yurimaguas through the jungle. Needless to say they are complete shit and even in dry season theres parts where you have to get ur bike in up to the handlebars, since I haven´t encountered any bridges... well none that you´d wanna cross with ur bike anyway...

Just for the bike was by the way over 900US. I speak fluent spanish so I don´t think they screwed me over too much, but it may be cheaper. Coming from Brasil though, I´d be more likely to cost you more though.