I am fairly new to OsmocomBB. Been in North America I acquired a C118(850Mhz/1900Mhz version of the C123) and a C156(850Mhz / 1900mhz version of the C155) but from what I understand at this point is that only 900Mhz and 1800Mhz are supported.

Yes. Most dev being in Europe we never tested anything else. But "most" of the code should already be ok (just not tested), but not all of it.

I am trying to figure out what modification I need to do in order to implement North American support for OsmocomBB, do I need to modify the firmware or do I only need to modify the host application? Any pointer would be greatly appreciated. I tried to modify the host application but I didn't had any success so far...

You definitely need to modify the firmware as well. At the very least I see that the RITA PLL configuration routine doesn't support all the bands. There are probably other places.

Also I am trying to acquire a C123 and I find it quite difficult to find in North America, anyone know where I could acquire it from? I have look online but I could only find it on https://www.samstores.com/ and it seem a bit pricey and I have no idea if this store is trust worthy.

Definitely quite expensive ... I bought mine for 19 eur + 8 eur shipping (to EU) from 3gsm.de

But it seems they only ship to EU. You could still ask them tough. For a single phone, the shipping might be quite expensive ...

OTOH If you're in the US there will be no 900/1800 network anyway. And if you're "close" to the BTS (like an openbts / openbsc you run yourself), the C118 should work fine in the 900 / 1800 band (just choose either a low ARFCN in the 900 band or a high ARFCN in the 1800 band).

Cheers,

Sylvain