Where can I buy large amounts of ETH?

Could you please enlighten me on where exactly I could purchase substantial quantities of Ethereum, or ETH, as I'm interested in making a significant investment? I've been exploring various options but haven't quite found the right platform or exchange yet. Could you recommend any reliable and secure sources for such transactions? Also, would it be advisable to split the purchase across multiple exchanges or stick to one reliable platform? I'm eager to learn more about this as I'm new to the cryptocurrency scene.