Here is a list of retailers that will ship to Guernsey addresses. Please note that many retailers will not explicitly mention Guernsey on their websites but you can confirm if they deliver by entering a Guernsey postcode. This page also includes a useful search box to search all the retailers at once.
Aside from the retailers shown in the table above and included in the search, another tip is to use the Shopify search function. This searches a wide ranges of Shopify that deliver to Guernsey. Make sure GG is shown in the filters and if not add your postcode.