For small items and small quantities...like "hard drives, RAM, monitors"....yeah whatever you find cheapest..such as Amazon or NewEgg. You're doing this past time and you mentioned "computer repair"...so those should be fine.
As you grow or take this on full time, that's where I'll encourage you to get setup with "reseller status" with certain brands of computers and other products. Yes your "cost" may be higher initially...but over time, as you build your history and quantity with a wholesaler/partner...your discounts get deeper..and deeper...and deeper. After some time your WAAAAAAY ahead in getting low costs than you would have been just buying things off the street like anyone. We started with CDW and Ingram...and a few years ago started using D&H for more...as Ingram started slipping in customer service for us. We're "MSP for businesses only"...so we use Tier-1 computer brands (Dell, HP, Lenovo), and business grade network products (Ubiquiti, Fortinet, Untangle firewall, HP/Aruba, etc) Many of those do require signed up/being approved for reseller status. We have several "backup vendors" also to source from.
For basis things like "buying 6x Western Digital Blue Solid State Drives"...yeah Ingram won't be the best price. But when you need to go buy a custom spec'd HP ProLiant or Dell server that has a cost of around 8 grand...you will find best pricing, pre sales support, etc...from a wholesaler. By far. We spent around 170k at Ingram, about 50k at D&H, and we still order direct from Dell sometimes...spent about 40k there yet still get a lot of Dells through Ingram and D&H. I use a really good resource located about an hour from me in Trumbell CT for my Lenovo products....did about 90k there this year.
For more full time business with steady products...it's about building relationships with vendors and wholesalers/suppliers..,..and getting line up with reseller programs.