External power supply for Arduino and other modules.

I am using Arduino Mega , Ethernet, SD, SIM800l, RTC, thermal printer, display module for a data logger. But cant find a good low cost power supply regulated on 5V. I have tried chargers, led strip power supply but noone is 5v plus minus 0.2V. Many are drifting over time. I tried led power supply 5v(5-7V) with buck converter 5v it is still failing to 5.5V. The only reliable is ATX power supply but it is too large and waste of money. What power supply should I use. The device have to work at least for 5 years.

You have quite a lot of high current devices in that list - a GSM module (several amps peak), thermal printer
(probably the same). It may be you aren't using thick enough wiring for your supply wiring and ground
wiring, or inadequate connections/connectors, leading to voltage drooping at the point of load.

Consider using several DC-DC converters all running from 12V or similar - the cheap adjustable LM2596's
you can get are worth considering. Ensure each high current load has its own DC-DC converter and you
should isolate the other devices from voltage dips. And you can use thinner wires from the 12V supply
as the current is less and voltage dips on the 12V don't matter.

On sleep mode they consume some mili amps.. I dont think it is about voltage drop. Voltage regulation is very bad.

I use an “Anker 15000mAh External Battery Pack Portable Charger Power Bank for USB Devices.” IOW, one of those USB backup batteries for charging phones and such. The Anker has two USB ports, one is 1A and the other 2A; the 1A port is perfect for running an Arduino Uno for hours or maybe days at a time. Just use a USB cable from the battery to the USB port on the Arduino.