The injector you have in the link looks like part of unionmedicos disposable line. I think they are designed more for vaccination. The injector I used to use is this one:
This allows multiple use. If you do go for this one be aware, the size of syringe you choose when purchasing is the one you will always have to buy from then on. They have a guide as to needle size, I inject in the thigh and have quite large thighs so went for the 1.25" which is fine for me, but some people will only need a 1" needle.
It helped me in the beginning with hesitation when injecting, so is a good place to start if you have any needle phobias or are unsure of your injecting skills! Only thing I would say is the initial injection does sting a bit more than if you gently inject yourself, mainly due to the force it gets thrown in at, but you still have control of the plunger, so can take it nice and slow at that point reducing the discomfort.
Regarding ampoules, there are a number of posts about the various German pharmacys that sell, I have used Versandapo and MyCare, both succesfully. Although MyCare charge a bit more for shipping.
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