Lot of time we need to use a function which act like "Like" Or "Contains" in ssrs.
Here is a small snippet which will show how to use InStr and Value.Contains function in ssrs to search part of string.
=IIF(InStr(Fields!ColumnName.Value,"a") > 0 , True, False)
Above expression will return True if a found else returns False, similarly we can use another expression as shown below
=IIF(Fields!ColumnName.Value.Contains("a") , True, False)