Check the yellow pages in your area for party rentals-they do rent all sizes of dance floors & deliver & setup. I cannot think of any other alternatives-as you'd need to 'rake' the sand to really level-and then-maybe place firm lumber/patios stones imbedded in the sand?

You may be able to rent a "Marley" floor. My DD's school of dance used to use one. You may be able to borrow one -- don't know how careful you have to be with it -- this may not be possible. If you have a tango group in your area, they may be able to steer you towards a source.

Here is a link that might be useful: Marley floors