Just check numbers from various likely-looking parts. If it has an IR with a diode-looking symbol inbetween the I and R, there's a good bet (International Rectifier - they do make other things, but they make a lot of MOSFETS, some in packages you might not expect). There's half-a-dozen duals in 8-pin packages on a particular flavor of dead hard disk board I have, but those are SMT, and non-SMT hard drive boards are probably very old, IME.
If you find a Philips part, you need to look at NXP for the datasheets, as Philips does not even redirect you there, or admit that they ever made parts on their website.
yes the non SMT drives are very old indeed 340 meg scsi , the PCB was a lovely foray into SMT tecnology with leed-less resistors and small tiny wire wound inductors in tiny plastic box mounted to surface of PCB with many other SMT sized ICs
RF mosfets aren't going to be common. I wouldn't even say they were dominant at any point, but they were part of a learning curve and about forty years ago when they became available to end users they were perceived as the next best thing, with low noise and good ability to withstand overload and relatively high frequency response. But oddly, right about the same time the designers were realizing how to properly use bipolar transistors, tossing out the notion that the best use for transistors is in low current use, so bipolars started their rise as good devices for rf stages.
Now, very few RF mosfets are being manufactured, and while at one point you likely could have easily found them new, the sources have dwindled at least in terms of the hobby market.
Which all means that finding equipment that uses them is going to be lengthy.
Almost a decade ago, I got some mosfets, something like the 3N200, in "FM preamplifiers" intended for use with car radios. They were at the dollar store, I bought one for the dollar, opened it up when I got home and found that it was a mosfet. I went back and bought the half dozen that remained. A very lucky find.
The likely most common place to find them, and I don't know how common they are used nowadays in the application, would be tuner modules in tv sets or VCRs. Open up the modules, and if you see any four leaded devices, then they are worth taking a second look at.
I suppose some FM radio tuners use them still, but I can't say I've seen them particularly often in such radios. Not likely in cheap portable radios, but in car radios and more expensive FM tuners and receivers. But again, there are other devices so you may have to be opening some or many to find the mosfets.
After that, the most likely places would be two way radio receivers, but even then it's a roll of the dice. I sure don't see them in
49MHz walkie talkies or cordless phones, and I don't remember noticing any in the cellphones I've taken apart.
It drops off after that, since you are less likely to come across esoteric equipment that might use mosfets.
You'll find power mosfets in various things, but again nothing depends on them, and their specs may not make them suitable for RF use.
One place you can get them in small quantities, and other interesting RF parts at reasonable prices is
but I haven't ordered from him in over a decade so I don't know how good the service is nowadays.
In the end, you can probably get by with a jfet buffer, it will offer a good enough input impedance and many a frequency counter and oscilliscope input have been made with them.
I can't say you'd find them that much easier in everyday equipment than mosfets, but maybe a tad more. I've certainly found them in Delco digitally tuned car radios, the big bulky ones with green displays, as the first stage in the AM section. I suspect you will find jfets more common in AM/FM receivers than mosfets.
In general, yes, the trend goes towards BJT because they can process much higher frequencies. ft around 50GHz is quite achievable at low cost. However, you can still buy lots of RF MOSFETs, mostly dual gate. Just go to Digikey, key in BF9, then select RF FETs and you'll see that they have five digit quantities in stock on some. For example 15,000 for the BF908. Take a look at the BF998, it's a real rocket.
Those things are really handy in building adjustable phase shifters and such.