where can you get upgraded injectors for the 4.3 or can you?

Really depends on what you have. Any of the modern trucks, no go. The CPI and SCFI have no aftermarket as far as injectors go. It is due to the configuration of the injection system. If you have a 96+ SCFI, then you could get a marine intake which would give you a much more boost friendly intake manifold as well as standard style injectors which can be swapped out for larger ones.

where do you get the marine intake man.

eBay... A little searching goes a long way.