How do I get allocated a subnet for IPv4?

It depends who you are and what your Internet connection is. Usually you rent it from your ISP or hosting provider; prices vary but it should be less than $1/month/IP. Note that you don't own these addresses; you're just renting them from the ISP that owns them.

If you are an ISP (which I'm guessing you aren't) you can get them from ARIN/RIPE/APNIC/etc. or buy them which is a more complicated topic.

I've got a few class Bs in the 192.168.x.x range I'd be willing to sell you.

That's amazing, considering every network I jump on gives me an address in this range.

That's why they don't come cheap. You want to buy a subnet you can count on.

https://www.iana.org/numbers