The boiler to water/air heat exchanger is the way the outdoor wood boiler folks use their boilers. if you search on ebay for heat exchanger you will find several dealers that sell them.. they need to be the right dimensions for your hot air plenum..
The neat think about a boiler is that you then can easily heat your domestic hot water with the boiler,, eliminating any oil use during the winter months..
For boilers.. Keystoker, EFM, Harman, AHS, Axeman Anderson, and a few others I can't think of right now..
There are dozens of threads on the forum on this very same subject.. sit down and so some searches.. read several pages back there is a wealth of information available for reading..
Hope this helps..