I don't shop them there fancy high-end boutiques. Amazon.
How do you find anything on Amazon - they are the kings of fluff and distraction? I get very frustrated there. Their search algorithm is designed on purpose to show you random stuff, or things that are only peripherally related. I searched for "M4" tap and got M40, M4.5 and a bunch of other silly stuff. McMaster's website is world class and gets you to what you are looking for very fast, or a total blank, which is ok. Better than an Amazon search that yields 100's of wild goose chase hits.
I did find the cheapest 10-24 tap of HSS at $4.77 with free Prime shipping from Drill America, the other cheaper ones were over $10 including shipping. Once could debate whether Prime free shipping is free or not, but if you have a membership, yeah you saved 64 cents. Beats me if you really get a genuine Drill America drill either.
Found a M4 Drill America tap in HSS for $8.92 with Prime. Some of the other taps that might be cheaper don't seem to be in the same quality league.
These prices are only
slightly cheaper than McMaster. The quality of the parts probably is significantly different as well. Understand why you are looking there. Sometimes I check Amazon, but often find a better deal elsewhere, sometimes even at McMaster.
At least McMaster has
better than Prime shipping. Amazon has reneged on 2 day shipping for Prime. I often get orders from McMaster in 1 day.
If I can wait, sometimes I'll go to eBay. If I don't care if it will take quite a while, I'll try AliExpress. Never been burned there, but never have ordered something really expensive. Got some diamond stones 70 x 170 mm for $2 each in various grits from AliExpress. They were worth the wait. Amazon was selling them (at the same time) for over $10 each, with long delivery times, straight from China.