PFL's, we had some hard wearing water proof covers made out our way, cost was $300 for a cover that could cover a front loading wasing machine.
You'll generally find that, where there are large concentrations of hardware stores, there will be a dude with his sowing machine, making weather proof covers.
You could try:
Shanghai & Bute Streets Mong Kok
Lockhart road Causeway Bay/wanchai
In and Around Wang Fat Path Yuen Long
Toward the old part of Sai Kung Town and also the outdoor furniture shop there, a few doors down from 7 Eleven, in the main drag.
That's about it, as I don't know where you are located to offer any further advice...
Have fun lol