Unfortunately there isnt much better alternetive to flea treatment than the product that you are using.
Many people have found that alternating from month to month between Frontline and Advantage helps to control the fleas better, as they both have different active ingredients- so the fleas cant get immune to them. Sometimes when you use the same product for years on end they can start to get immune to the treatment- so this is a good method to prevent this problem.
I dont think there are any natural preventetives that work better than this. Garlic is supposed to help prevent fleas- but honestly- the effects are minimal. Have a go with Frontline one month, then Advantage the next and see how that goes.
As you are already buying the large tubes of treatment I cant recommend buying the large ones and changing the dose, as you can with the smaller dogs.
You might want to try buying the treatment in bulk from a website mayve- although this stuff can be difficult to get. If you have an ABN number, or you are close with your vet or good pet store owner, you might have more luck at buying it at cost price, but I dont think it will take the price down too much... Good luck!
If your dogs are still itching after an Advantage treatment it might be worth having them checked out at the vets for allergies or the like.