Try Medicanimal at http://www.medicanimal.com/product/~product_id=10127 I get mine from there, good price and fast service, and in stock it seems.
The pack will treat 20 chickens so will do yours for the next ten times, ie 5 years worth, so not so expensive as it may seem!
Your two will probably eat a little over 2 kilos in the week's treatment. Weigh out 2 kilos of pellets. Put a couple of handfuls in a large bowl and add a little vegetable oil, stir to just coat the pellets. Measure out and add the right amount of Flubenvet with the scoop provided in the pack, according to directions. Gradually add the rest of the pellets, stirring well between each addition until all are mixed. Empty the feeder and refill with treated pellets. Feed ONLY the treated pellets for one week, nothing else, then each bird will get the full and correct dose for her weight. You may need to mix some more at the end of the week, depending on their appetites.
I'm glad there's obviously been a national drive to worm chickens - maybe getting them fit in the wake of the Olympics?
Which breed gets the gold for laying, I wonder?