What your looking for is an older 1 1/8th straight steer tube fork.
I like the older Marzocchi forks. They are very smooth and you can find them on fee bay. A good 90 or 100 mm fork will be fine for your frame.
Depending on your budget theres plenty of options out there. You may want to be careful of older elastomer forks, they tend to dry out and not have much in the way of travel or rebound control.