How to buy more iCloud storage without an apple device?

Hi, I would like to know how I may purchase more iCloud storage without having to upgrade on an apple device such as, an iPhone, ipod Touch, ipad or a Mac? In the past I have owned an iMac G5, ipod click wheel, 1st generation ipod Touch and a Macbook Pro (2006). All are vintage now, but I am still using my Apple Mail, but on an android phone at present - Sony Xperia Z5, through an email client. Some of my old devices are still working, but I am having a problem at present opening up my Macbook Pro (2006), which features Snow Leopard OS X 10.6.8 version. Therefore, I cannot follow the normal way of upgrading on an Apple device with my Apple ID. I really need however, to buy more storage space soon because at present some of my emails are not coming through to me because I have been notified by Apple iCloud service that I only have 20 Mb of space left, so I want to upgrade to 50 GB of more storage space, but I don't know how to do it without an updated device or for that matter, without my currently non-working Macbook Pro. Therefore, please can anyone out there help me or direct me to Apple support? For some reason I can't get through to Apple's contact support page on my Sony Xperia Z5 either?

Thank you and kind regards,

Mandy Morris