Some of my favorites have been -
Modular Addict - 9mm Alpha Pots, Knobs, DIY PCB’s, etc.
SynthCube - Obscure Jacks and other various Modular Supplies.
SparkFun - This is specifically where I get my 2 Pin Molex Connectors, but they have tons of other stuff as well.
Love My Switches - I often buy the TruConnect 1/4" Jacks from here.
Perfect Circuit - Great for Eurorack 16 to 10 Pin Cables, and other various supplies.
There are some obvious others for really common parts, like Tayda, Mouser, etc.
For IC’s I often utilize Amazon Prime and Ebay.
Over the last year, having Amazon Prime has probably saved me over 300 in shipping on random parts orders… lol
Hope some of these help others!