It might seem like I'm a bit paranoid about single-source parts like the NXP BSS83. When I decided to actually build the discrete-part 4004 CPU, one of the first things I did was order enough of all the parts I'd need to build the whole project, plus extras for goofs. Since then I've received several courtesy notices from DigiKey about parts becoming obsolete, but all of them would be easily replaceable if I'd needed them. But I already have everything except the PCBs.
I have another project going which involves very low power RF circuits. It's another one that I started a couple years ago and work on in fits and starts. My plan was to build 5 or 6 of these little devices, and I carefully acquired what I thought would be the critical bits. I'd had 6 PCBs made and assembled two. This evening I decided to assemble another three for a larger-scale test. The only thing I don't have on hand are the #6-32 nylon screws that hold the PCB to the plastic case, so I went over to my local big-box hardware store where I'd bought them before. They no longer carry them. They didn't sell enough of them to waste the shelf space on them.
Why nylon? Metal has a way of detuning tuned electronic circuits so I don't want metal screws that close to the antenna.
This is not a major catastrophe. I can get suitable screws elsewhere. The big-box store will even let me order them in reasonable quantity and deliver them for pickup at the store at no cost -- with a 10-day lead time. But I wanted them tonight.
The moral of the story is that it's a really good idea to buy everything when you start, because you never know what will disappear or become harder to get.