Is this for a male or female dog? (Depending on the sex, a different type of diaper is often more effective.)
And to be used to contain urine or feces? (The above only applies in the first instance.)
Fressnapf carries a washable diaper that works well for female urinary incontincence, but it really isn't designed for for male urinary incontinence. It's made by Simple Solution, here:
http://www.fressnapf.de/shop/bramton...re-hundewindel
Yes, this is the German site, as Fresshapf CH's website is not very helpful - but Fressnapf Schweiz does indeed carry them. They are usually in the aisle with all the other health care products. If your local Fressnapf doesn't have them in stock they can usually order them for you in a day or so.
You might need to buy one size larger for most effective protection, depending on the build of the dog. Don't just go on the size/weight chart, but measure your dog.
This diaper might not work so well for fecal incontinence, due to the tail opening. This could be a problem with all canine diapers - if (medical) fecal incontinence is the issue, you might need to get a bit more creative.
For male urinary incontinence I have found that the most effective protection is to use a human disposable diaper as a 'belly band' - that is, turned sideways so that the elastic leg openings go around the waist and hips. Secure over the back using the tabs, pleat the gaping material and secure with a bit of masking tape. Larger baby diapers work for a small dog, you likely need an adult incontinence product for a larger dog.
One can get purpose made belly bands in the US that function the same way, but I have found them ineffective in most cases as the material is not absorbent enough. But if you want to go that route, Drs Foster and Smith carry them: http://www.drsfostersmith.com/produc...id=24773&rel=1
ETA:
Just noticed that Foster and Smith now carry a male wrap made by Simple Solution: http://www.drsfostersmith.com/produc...m?pcatid=27182
Since Fressnapf carry the female version from this manufacturer, perhaps they can order the male one if you need it. Worth asking (if this is for male urinary incontinence, that is.)
Hope a solution is found for the pooch you are trying to help...
ETA 2:
Just a thought - have you explored all medical options? Depending on the underlying condition, some cases can be treated medically or surgically. If you haven't already done so, a further chat with the vet might be advisable.