Your leads may not be the issue, maybe you just need to do more. One thing that stopped me from doing more is handwriting the letters. I found a company online that will actually create a font for typing letters up that is your hand writing. That greatly increased my efficiency with writing the letters. I think they charged me $10 to convert my handwriting into a font, so it was well worth the investment. PM and I'll give you their site.
If you are looking for better lead sources, I normally use services such as Landvoice Pro or just go to the foreclosing attorneys' websites that foreclose properties in your area (those people really need you!).