I've been looking into this and found the following.
1) Hella do two types, the jet single speed and the turbo twin speed. These seems to be used on boats, but are the ideal size for the car. Bad news is the reviews are poor. The price in the UK are high at £89. Best to go on eBay and get one from the states, where they pay the import tax, working out at £65 delivered for the turbo.
2) the durite one mentioned is on eBay. I might go for this one.
3) another idea is to use a computer fan of the same size (about 6 inches) these can shift bigger volumes of air than normal fans and can cost as low as £5...
also they can run on bearings, not brushes, which appear to last longer with a drop of oil now and then.
Hope this helps...
...... Which was nice.