Njoy Pure Plugs. They're made of surgical grade stainless steel. They're easy to clean. The most important things is that they have a great design and will stay in place and not slide out until you pull it out. Also the handle does not get in the way when you're having vaginal sex.
If you are new to plugs and you a looking for a safe material, try any small plug made of 100% silicone. The flexible, soft texture of silicone will make it easy for you and your wife to ease into using plugs. Plus, silicone is easy to clean. Remember to use a water-based lubricant, specifically one that is thick.