I always search around popular online sites like Qoo10, Lazada, Carousell.
Shops like itsmine.pc, a shop from SLS, is great for buying PC cases, peripherals and fans. They are cheap and you can get them delivered to you on the same night. Got my Thermal Grizzly Kryonaut that is rare in local online stores for way cheaper along with my desktop cases.
endless.passion is great for buying PC peripherals too. Bought mousepads and mice for way cheaper than retail and even Qoo10 discounts. Such as the Logitech G304/305 mouse at only $70 with free same day shipping as well. And they even give a free mousepad along with the mouse as well. They even have monitors and speakers too if you want to have one.
Both, I bought via their Carousell page since it is way cheaper even when taking Qoo10 with the discount coupons into equation.
As for Motherboard and CPU combo, only then it is advisable to buy together to get a cheaper price for the 2. Best to buy it at the shop itself.
It is the same for Graphics Card too. Though I bought some GPUs online as well via Carousell again. But it is the SLS's shop own Carousell page. Not some random stranger's page. But nothing wrong with that. I sold my GTX 980 Ti for only $400 bucks back then since I'm the opposite of a money hungry Carouhell user that feels like the value of my products are affixed at retail prices even though I've been using it for a year. LoL!
Hope my advice helps.
Most of my other parts except for the Mobo+CPU were bought online including the RAM. RAM usually have long warranty periods so buying off someone who has the receipt will be of no issue.