There is no special code for 'beginning of each cell', but there is a good alternative.
\A is the code for 'start of story', and each cell inside a table is considered "a separate story" -- just like footnotes, by the way.
This works best if you consistently used a certain paragraph style for just your table text; then you can select this paragraph style in the "Find Format" dialog (or any other formatting, of course). If not, you'll have to manually step through Find/Change, skipping actual start-of-stories and footnotes.
\A I know that one but it does not work in the latest InDesign CS6.
This is why I have created this one
it looks for any digit preceded by the beginning of a paragraph
One or two years ago I have found on the Internet a formula that finds the beginning of each cell but I can't find it anymore.