Lightning is indeed NOT a fork of a Bitcoin, it's essentially a Layer 2 for Bitcoin (if you think in terms of the Internet's TCP/IP model). It solves pretty much every use case created by any Altcoin when competing with Bitcoin, as Bitcoin chose security and decentralization over scalibility in its base layer (on-chain).

As for buying Monero, I only deal with Bitcoin, but I'm pretty sure you can easily buy Monero still as long as you don't use centralised exchanges (permissioned), and instead use Decentralised ones (permissionless). That'll be the case for any altcoin that still has decent popularity for the foreseeable.