Hi, I bought the following bundle, but I need to purchase the silicone strap included in the bundle again. However, it is not available as an accessory on the Garmin Shop website. What can I do? thank you. regards

So what exactly do you expect that we advise you? If that specific model is not available, using a different one is the only possibility, I'd tell.
I see minimally 3 straps in the store that look pretty much the same (below the 22mm versions, since you did not tell what your watch size is):
