Sell some unused cars.
You are probably going to just have to keep racing the same events to get more credits. It is lame but just think of it as a way to test out your new cars.
Every GT has had a grind if you wanted to buy new cars and / or modify them -- not sure what the surprise is, unless GT5 is your first game in the series. There's nothing like an oval for fast credits. Indianapolis Speedway in Like the Wind is good for 70+k and, when you can, the lvl 22 American championship is 90+k. Until you get there, Daytona Speedway -- not in NASCAR's challenge, as they cheat -- in one of the earlier levels deals decent credits.