A good quality alu post can be had just about anywhere for about $15-20. You need to know exactly what size tho. Digital vernier is best to measure this. A post that is .2mm off either way will not work. Was the old one really corroded? If so it might be the right size and there is a corrosion build up that is making the correct size post too tight. A flex honing of yer seattube will remedy that (or some sandpaper wrapped around a dowel if you want an easy DIY) Then you can just use the old one for sizing.