Hi clee123,
If you are looking for 2rd hand UPS, try defu lane 3 look for a kal-lan-gu-ne collecting centre near the bend. The boss is known as "Ah Seng". Dark, short and Skinny man. Wearing Dirty cloths, but easily carry more than 10k in pockert. Use to sell him old used UPS. Should be less than $50 for one medium size. Check the battery first.
Another place is Upper Dickson Rd look for a shop called Triumphant which sell sec hand computors. Look for the Boss Kalam. Short and fat Bungaladiesh who wear simple cloth but another one who has more than 10k in pockert. This one has an empire at home country. He should has some UPS. Price will be more expansive than Ah Seng.
While at defu, you might be able to find your portable gen set. I once brought from him a 1/2hp chiller for $20. IT was still quite good except that has no casing and controller. Other things like CO2 canistor or Filter Canistor can be found there too. But it all depends alot on your luck and your know how if you bought one that is not working properly.
Good luck.