PCB designing of GSM module
The reason is usually that the manufacturer doesn't have an arrangement with distributors (eg. Farnell, Digikey) so you have to buy from them directly. They do not care about someone who wants to buy one or two, you usually have to be talking about regular orders of 1000s of pieces for them to bother with you. Clearly ITEAD are able to order a large enough quantity from Simcom. Additionally the SIM900 is at end of life, they suggest using the SIM800 instead.
Is it feasible? Yes, but... Guessing from the level of your question (apologies if I'm mistaken), you wouldn't have the experience/tools to be able to design a good product. I would suggest getting a module with the antenna in place, and just the digital control pins (and power) brought out.