I can tell you where I buy from usually. Newegg, but I would recommend not looking for cheap. The power supply powers everything in your computer. If it goes out it can take motherboards, hard drives and video cards with it. A good quality PSU is always needed. I always recommend and buy either Corsair, Antec or Seasonic PSUs.
Please fill in your specs. It helps others help you.
300W power supply is very low for today's computers. Once you fill out your specs we will able to advise you better on power supplies.
My Computer
At a glance
Windows 10 Pro. 64/ version 1709 Windows 7 Pr...Intel i7-6800K @ 4.3Corsair Platinum 16 gig @2400EVGA GTX 1070 OC
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Home made Desktop
OS
Windows 10 Pro. 64/ version 1709 Windows 7 Pro/64
CPU
Intel i7-6800K @ 4.3
Motherboard
ASUS X-99 Deluxe II
Memory
Corsair Platinum 16 gig @2400
Graphics Card(s)
EVGA GTX 1070 OC
Monitor(s) Displays
Asus 27" LED LCD/VE278Q
Screen Resolution
1920-1080 or 1280-720 HDMI
Hard Drives
INTEL SSD 730-240 Gb Sata 3.0/
PSU
EVGA Platium 1200W
Case
Phanteks Luxe Tempered Glass 8 fans/ one radiator
Cooling
XSPC/ Water Cooled CPU
Keyboard
Das 4 Professional
Mouse
Logitech M705/MX Anywhere 2-S
Internet Speed
100 mbits
Antivirus
Microsoft Security Essentials/ Malwarebytes Premium 3.0/ SAS
Browser
I.E. 11 default/Firefox/ ISP Time Warner Cable/Spectrum
Other Info
LG BluRay Burner/
Sound system-KLipsch-THX/
Icy Dock ssd Hot Swap bays.