Yes Frontline seems to be losing it's effectiveness as fleas are becoming resistant to it. Advantage or Advocate are good. Do not buy cheap off the shelf products. Not only do they not work, they can kill cats. I volunteer for a cat charity and we have had a lot of cats brought to us, dying, because their owners used off the shelf flea treatments which their cats reacted badly to. (Convulsions followed by death - we manage to save about 25 per cent if brought to us within the first hour - otherwise they have all died). Bob Martins has been particularly bad for this, although I believe they have very recently changed their formulation. But most other cheap off the shelf products contain the same, dangerous ingredients.
Try online, but if you are buying without a prescription, don't bother with anything other than Drontal (for worms) or Advantage (for fleas). No point throwing money away on something that doesnt work. You dont need to treat them for fleas if they dont have any (unless you want to prevent infestation). Buy a flea comb and keep a regular check. There isnt any point in treating every few months for fleas - either treat all the time so they dont get any, or just wait and watch and see. How often to worm depends on your cat. If they hunt, try and do it monthly if you can (which I know is a nightmare). If they are primarily indoor cats, once or twice a year, and during flea infestations if you get them, is fine.
Was wondering whether there was anywhere other than my local pet shop to buy Flea/Worming Tablets as I have 3 cats so it can be expensive to treat them all , Just wondering if there is a cheaper way to get them