Hi Aaron,
here is a link I found where you can buy Lysine
http://www.nutrisun.co.uk/search.asp?wh ... ZgodPlJI6Q
Only thing there is I wouldn't know how much to give a cat without over dosing them !
we feed our cats on Royal Canin complete dried. we feed kittens on RoyalCanin Baby Cat and this food contains Lysine.
This is a link to Royal Canin and there is a telephone number on there and you could ask them which food is best for two 10 month old Bengals.
http://www.royalcanin.co.uk/
We feed our cats Whiskers or Felix once a aday just as a small treat. There isn't as much goodness in the tinned foods, it's mainly water. We much prefer the complete dried as it also keeps their teeth clean.
What food are you feeding them on at the moment ?
It could be worth switching their food.
You are probably guessing correctly that your cats picked something up whilst in the cattery. They can harbour viruses for months without showing any signs of illness and thern suddenly start to be ill.
I do say they need the correct antibiotic. Sounds like the antibiotic they've been on isn't strong enough to cure it.