I buy most of my parts from Mouser. Occasionally I'll use Digikey, then Newark. For more industrial electrical parts, I use Allied, then Grainger.

Some stuff is getting hard to get. I ordered 500 bulk transformers from Mouser to be drop shipped from the "factory", but they were backordered for months. I finally bought 25 in retail boxes as I had run out, and the 500 arrived the day after with no notification. Well, 2 days later I did get an email saying they had been shipped...