These ring vibrator are sold over-the-counter at many pharmacies. Although they are intended to be slid over the erect penis for stimulation, that does not help to delay premature ejaculation,
One technique that may be helpful, however, is for the female partner to slide the ring over her finger and hold the vibrating ring against the perineum of her partner. (The perineum is the area just forward to the anus.) The prostate gland is just under that area and will also be stimulated. The vibrator may also be held over the anus (without inserting) to cause additional stimulation.
The rationale for this technique is to stimulate areas other than the penis to cause distraction during intercourse and prevent the male partner from getting so excited that he will reach climax early. This is a very effective technique.
Below is an additional picture of the ring vibrators.