it appears that your requirement is for the width and not the height or length...??
have i got that correct...??
after a quick look for sources in the UK try
https://www.hengehammocks.com/ as they have a section where you can make your own gear and their gear includes hammock spreaders on ebay albeit 32" not 36"
https://www.ebay.co.uk/itm/164366765735
other than them in the UK i do not know where else you could get them custom made
i think the alternate way is to [DIY] a custom pair or set for each hammock
they can be made out of most materials such as wood plastic aluminium and-such