No, you can't buy that service.
Not even close.
400amp service is usually 2 200 amp panels.
Done with 2/0 cable. It's rarely, if ever, 1 400 amp panel.
1. There is no way you would get "1000amp service" at 250v. It would be a minimum of 480v 3 phase (and not 1000amp). You may even want a lot higher, because as you'll see, getting that many amps is ... very very hard.
2. You can't even buy 1000amp cable gauge anywhere commercially, because it would be insane.
The NEC ampacity table doesn't go anywhere near that high
it tops out at 2000kcmil, 665 amp cable.
Due to various factors[1], doubling the cable mils will only get you a hundred or so amps here, so it's probably close to 4500kcmil (i'm too lazy to do the mm^2 math, it's 1.89" conductors)
Which doesn't really exist for purchase (if you really needed it, you have the means to make it :P).
To try to also put the size in perspective:
2/0 cable has conductors that are about 3/8" in diameter.
2000kcmil cable, which again, tops out at 665 amps, has conductors 1.6" in diameter (they are often also segmented, so while the bare size may be 1.6" in theory, you don't find that).
Generally the insulation size on them is ridiculous as well (because they are usually used for like 10kv+ applications)
[1] Current is related to surface area of conductors, or combined surface areas of all the strands in a conductor. So you'd need to double diameter, at least, to come close.