Newplast is plasticine is non-drying modeling clay, newplast is just a specific brand of plasticine/modeling clay chosen because it has a slightly better damping coefficient, the differences are fairly minimal, and I would only recommend newplast over the non-drying modeling clay if you wanted the absolute best at all cost. Not worth it for anyone else IMO. You can find modeling clay at your neighborhood Michaels or Staples or hobby store. Newplast is UK only, you would have to order it out.
akasa is worth the price IMO, and you get alot of it, enough to mod many headphones.
Also if you are cringing at the price of the SA5000 pads, try the srh840 pads, for $20 you get some good pads, they are pleather, but very comfortable, easy to angle, and create a miniature room between your ears and the driver (helps soundstage IMO). ebay has them (he ships very quickly) as does amazon. If you do not like them, then spring for the SA5000 pads.