Williams Foolproof FP-88/100, either with screwdriver adjustments or you can get the target knobs. OR, the Williams WGRS-100, which is the guide receiver sight. It doesn't have the click adjustments which the Foolproof has, so you loosen the clamping screw, slide the aperture up or down (or left or right), then tighten the screws again.
I have used both on my 88, and the Foolproof is what I stayed with before my eyes required a scope. The WGRS is more compact and less expensive, though.
-WH-