I have had to buy new clothes very early on as i started to show straight away. i bought some maternity pants from a maternity shop and they had a belly pillow so you could see what you would like like in the cloths you bought. they were all ajustable so as i got bigger i would cut the stitches to let the elastic out. i have out grown them all now but have found the lycra maternity pants very comfortable and they have a fold down top which is good as some of my shirts need the extra bit.
As for maternity bras I have read that the underwire bras can harm the breast tissue as it gets ready for milk production. not sure if its true but i was a B/C to start with and now a D/DD and i still have 13 weeks to go. they are very confortable and there are a lot of differant tyoes now out there including Tshirt bras. I would just recomment that you try them on first and maybe buy the next size up