Take to any local shop and buy the RAMs from them,
I just bought 512MB SDRAM for P4 last week on it sticker of IBM was there.
You can try brands like Hynx(earlier when IBM and not by name of Lenovo used to sell computer products Hynz were the RAMs it used), Corsair, OCZ, get the RAMs checked the same time. Its better to do this in the shop itself