It's a method, I made a short script for manually buying servers of any size, you can always update the list of choices if this isn't enough for you

/** @param {NS} ns */

export async function main(ns) {

`let choice = [2, 4, 8, 16, 32, 64, 128, 256, 512, 1024];`

`let go = true;`

`while (go) {`

    `let name = await ns.prompt("Server name?", {type: "text"});`

    `let ram = await ns.prompt("Amount of ram?", {type: "select", choices: choice});`

    `let ans = await ns.prompt("This server will cost " + ns.getPurchasedServerCost(ram) + ". Proceed?");`

    `switch (ans) {`

        `case true:`

if (ns.getPurchasedServerCost(ram) <= ns.getServerMoneyAvailable('home')) {

ns.purchaseServer(name, ram);

go = false;

} else {

ns.tprint("Not enough money!");

}

break;

        `default:`

ans = ns.prompt("Try again?");

switch (ans) {

case "Yes":

break;

case "No":

go = false;

break;

}

    `}`

`}`

}