Weighted items, new and second-hand, are widely available from a variety of suppliers in a number of countries, including the UK and the USA. It is also possible to make some of these items at home, either from scratch or using patterns provided by suppliers.
There are a number of specialist suppliers of weighted products but there does not appear to be any industry standard that applies worldwide or even within specific countries like the UK or the USA. For this reason, we strongly recommend that you should consult an appropriately qualified health care professional (such as an occupational therapist or a physiotherapist) before using any weighted items.