Where to download user-created assets?

Hello. I've been using Source SDK for a few years now and am beginning on a new map which I will release later next month. However before I do so, I was wondering to where I could (if possible) download external assets (props/textures)? I see alot of maps with custom props, and, lacking the skills to create high detailed props myself, I have resorted to needing free/paid props from the internet. However... I can not for the life of my find anything which contains large amounts - I have found single downloads to crates, doors etc, but never any large collections etc. I was wondering where I could get a large collection of Source SDK models/props and/or where to buy them from. Thanks.