Are there any line in adapter alternatives apart from the one Sonos provides?

I don’t think so - as stated by @controlav above

So Sonos is willing to sell us a $250 or $450 speaker. But incapable of providing a straightforward dongle so a PC or Mac can send its signal to the speaker. But that’s the reason I bought the speakers.

Probably 75%, as a guess, will use them as surround speakers, in which the line in is disabled. Sonos is both saving needless expenditure, avoiding needless pollution, and keeping prices as low as possible. 

… and also avoiding their promise to make the speakers work as advertised.

I bought the speaker because of this function as well so really disappointed I have a 350$ speaker with no use for several months now if ever they have stock in my area. This experience alone makes me not like Sonos and I spent $$$ money to convert my whole house to in ceiling, surround etc. They should include a 20$ dongle and eat the cost as that is the main selling feature and why I bought the era 100 to connect to a vinyl record player and the rest of the system. 

@AdamRivard - I ended up ordering the $40 Sonos Combo Adapter from Crutchfield because they were the only company who had it in stock and who would ship it quickly to me. I wrote an e-mail to Sonos’ CEO about its lack of adapter availability and planning, from his own link on their website, and I never received a reply. Here is the link where I found his e-mail address (https://www.sonos.com/en-us/contact). I am currently using the Combo adapter to switch between the 3.5 mm output from my PC and the Sonos S2 software controller connected to Spotify. Although the combo adapter functions, I found it to be very cheaply made for something which costs $40, and I mentioned that to Crutchfield.

Fine, then allow it to work with any third party USB C to 3.5mm adapter, rather than force people to spend another $25 on something that isn’t in stock ANYWHERE!

Sonos is too good at making stupid, unforced errors like this lately...   

That’s the thing, third party USB-C to 3.5 mm adapters aren’t analog in and digital out, they are basically headphone jacks, i.e. digital in and analog out.  That’s why they don’t work, not because Sonos is preventing them from working, they just do the opposite of what the Sonos dongle does.  

I can confirm that the Apple USB-C to 3.5mm Audio Jack / Headphone Adapter does NOT work for line input with the Era 100. I bought the Sonos line adapter and it works. The line input source is automatically recognised as an audio component source. The Sonos widget is larger than Apple’s and better quality. There must be a chip in it controlling the handshake, doing impedance matching, and/or A/D conversion, otherwise there’s no good reason to make it proprietary. I agree with others that it should be included with the product.

Sonos should make it much clearer on their website that the Era 100 (and perhaps any of their newer USB-C line input devices) requires their line adapter widget! It’s about 2x the cost of the Apple USB-C/3.5mm jack, so I’m not happy. But life is too short to spend any more energy on this.

Don’t get caught out folks.

P.S. Why use line input? Well for me it means I can then use my Dolby Atmos Headphone sound controller on my computer to expand the speaker’s soundscape. Dolby Atmos only works with the PC’s headphone audio output.