Are you looking for a toy Planet Express delivery ship, or a model kit? I have seen toy PE ships online fairly cheap, and I just recently bought, online, a PE model kit from Skyhook Models. I believe the website for the model was www.skyhook.com.
But, I know you can find it by doing a search for "planet express delivery ship model kit". If I come across the toy PE ship that I saw, again, I'll get back to you with the link. In the meantime, if you should find a toy ship at a reasonable price before you hear back from me, please let me know where you saw it.