Perhaps try ebay? or do a google image search for "pickachu plushie" , then when you locate a picture that looks similar follow the link and see if you can find a distributor for the plush.
is there a tag on the plushie? Often that is a good hint on where to start looking.
Oh HECK! My long time friend has one of those but I highly doubt she will ever give it up so sorry Foxy. If you ever do find one,expect to pay around $70-$100 due to its rareity from age and supply.
I typed in pikachu plush on amazon
.com and a bunch of them popped up, not shur which one your looking for.I would post a link but I don't know how to do that.
At a glance, here's one. Or maybe this one. Actually that last one's pretty big. Well whatever...
Good luck! There appear to be more than a few Pikachu plushies out there!