Welcome to the AT forums.
At places like
http://www.sidewindercomputers.com you can often find direct replacements for Dell fans if the reason your fans are noisy is that they are just wearing out. But you need to know what you are starting out with (size, speed, current rating) to get something that will work in a Dell as they are usually tightly tuned to the OEM fan.
OTOH, you can use any fan that will fit hooked to a manual fan controller to get good results. So report back with the size(s) of fan(s) (make and model no. would be useful too - often you can see the labels on some of the fans from the back of the case) you need so we can give some suggestions.
.bh.