I laid some st. Augustine in the back yard in mid march. The yard was very lumpy beforehand, and I tried to smooth it out as much as possible.

But there are a few small spots that are still low.

What is the best way to fix this? The grass has come in quite nicely, is it too early to put some top soil on it to level it out? How much do I put on? Can I get it at Lowe's?

I wouldn't think it's too early. Just don't smother the grass. Put enough on to level it (again, without smothering the grass). Lowe's carries Black Velvet topsoil which is topsoil with some composted cow manure mixed in. Great stuff.

Thanks for the info.

If you have to cover some parts of the grass with soil it will cover it in time. It will look odd for a while, but it will spread. If you take the stolons (above ground runner) and cut them off your established grass and push them into the soil they will grow and cover those spots faster.