Shop around more, and check NOAH. The NOAH site will tell you what packing quantities each product is manufactured in, and the dosage, so you can work out how long each bottle would last you. Not all shops sell all sizes! I use the Better Returns wormer list document and go through each one and find out what sizes they come in. Then I look for a shop that sells the size I want, either online or your local agri store might order in the smaller size for you.
If it helps anyone, I use:
Group 1: Parafend (Oxfendazole) 1 litre You can also get Panacur in very small bottles
Group 2: sorry never used ..yet
Group 3: Noromectin (Ivermectin) 1 litre
Flukicide:
Triclafas (triclabendazole) 1 litre
Flukiver (closantel) 1 litre
Injectables obviously you can get in smaller quantities - but again shop around to make sure you are getting the smallest bottle available.
And of course check the use-by date when you buy - the NOAH site will also tell you what the shelf life of the product is which should give you an idea of what use-by-date you are aiming for. If you buy from the bigger online stores they sell a lot so you usually get one with a long date - but best to ring and confirm before ordering if you can.