Are you looking to make a new Kroot species? If so, and you're sold on the "arachnikrootaur" idea, then I would recommend the Fantasy Goblin Spider Riders' spiders as the basis for your conversion. They can usually be obtained fairly cheaply in groups of 5 or 10 on eBay. When those figures first came out I remember seeing a Centigor conversion using these guys and plastic gors, which looked really cool!
It will require a bit of modelling though - you'd have to file down the facial features of the spider, and on top of that you may want to add loincloths or belly plates to cover that gap if it proves hard to cover up with green stuff. Generally though, I think these two models (spiders and kroot) are very compatible, aesthetically. The spiders have the same harsh and pointy features as the kroot, and some of them have similar little spines or quills on their bodies.
If you're still looking for other suggestions, what about some serpentine kroot? If you're confident in your GS skills, you wouldn't even have to buy any conversion parts! Just take Kroot torsos and extend them into a long snaking tail.