Cross refills are quite simple to choose replacements for, but have several variations to decide from, and you cant just rely on the type of refill, for example whether or not its a ballpoint pen.
Each of the pen refills should have details on the product page about which exact pens it fits, and you should also be able to compare the old one against the photo (if you have it). If you still cannot find the right one, or need more help then contact us and we will be able to offer expert advice.