I think you're going to run into a steep price anywhere you look, unless you buy it used. PC component pricing, particularly with DRAM like your current DDR, can follow a reverse bell curve. Price is high when new, tapers off as newer components come on the market and drive the price down, then gradually rise again as manufacturers stop making it and the available supply dries up. Very few memory manufacturers still produce DDR2, let alone DDR like yours.
If you're fixing it up as a personal project or for sentimental value, it's not going to be cheap. You can find stuff on eBay, but there's always the chance it'll be faulty or DOA. If you're looking for actual performance, it'd be cheaper and easier to build a new system and performance would be infinitely better.