Where to get X-Large baseplates

I'm planning on doing a massive display in the near future, but have found that there are simply no used 48 X 48 gray baseplates for sale on bricklink and checking ebay it seems like the demand is fairly high for these. Is this a correct assessment? Is my only option buying new from Lego? Do they ever go on sale?

I'm very new to this end of the hobby but I'm seeing big bucks pile up when it comes to getting all the baseplates I need. Any tips would be appreciated!