For the linearity control, I wonder why you couldn't use a 5K pot with a 1k resistor is series? Seems like that would be a functional equivalent. 8 meg is going to be tough, other than another parts set. 10 meg are out there.
http://www.allelectronics.com/make-a...ear-pot/1.html
edit - just found this.
http://www.ebay.com/itm/1-Clarostat-...UAAOSwKPNTyrQw
If there's room to mount these type pots, I'd try it with a 1 meg resistor is series.