What are the best maps, if any, we have of the convection cells in the Earth's mantle?
If you're looking for a cool, well detailed map of convection cells that looks like a modern weather map....forget it. What you're looking for has been notoriously difficult to produce, if not impossible. Even the most advanced seismic tomography techniques using seismic wavespeeds can only resolve mantle "features" thousands of kilometers in scale, but really, only in the X-Y plane. These techniques do a poor job of resolving anything in the Z plane.

Even the locations of convection cell boundaries appears harder to find than theory suggests. Currently, the influence of convection boundaries has been mapped from the surface elevations of oceanic plates, but those maps look like odd "heat maps" indicating where oceanic crust has domed up beneath upward mantle flow. No vertical flow lines boundaries are even remotely suggested with these two-dimensional maps. You can read the actual paper published in Nature, 2016 here: Global dynamic topography observations reveal limited influence of large-scale mantle flow