Balsa wood and its different grades...

I've run out of balsa wood for making bodies. The stock I had was many years old and superior to recently purchased stuff. The recent stuff is incredibly soft so much so when I grip it with my fingers its leaving dents in the surface so not suitable. My old stock didn't do this and was harder.

Can anyone help me source a harder grade balsa? is there a spec for the harder grade? or a trusted UK supplier anyone can recommend?

Thanks.