My car is leaking transmission fluid on my 2004 diamanté ls. Are there seals and a filter to replace? If so where they located and how expensive is this repair?

A transmission can leak from several places. The transmission pan gasket, the output shaft seals, or the front pump seal are the most common. If the transmission pan is leaking, then that can be covered during a standard transmission service (filter and fluid exchange). If the output shaft seals are leaking, you can get that repaired fairly inexpensively. The front pump seal, however, is very expensive to replace. It requires the separation of the transmission from the engine. Have a shop look at it and have them give you a quote.