The closest thing to riot shields I can think of is the ballistic shields that came with some of the 21st Century Toys SWAT figures.
There was a line of 1/6 action figures made for a TV series here in the UK called 'The Bill' which was about the Metropolitan Police. One of the figures in that, called Public Order or something, came with a small circular riot shield. Not the long rectangle types, but still a riot shield all the same.
Not sure how easy those will be to source though.
EDIT: Here's the figure in question...