A cheap cosmetics mirror works great and are easily replaced with they start to wear out, or you can get a 75mm double concave lens for relatively cheap too (maybe $12-15).
There are glass lenses over at DB-Tops.com for $8.99, they have a optional silicone band to protect you and your top from any unpolished edges. I will be offering them along with my next drop. Nothing like having a brand new surface to spin on.