Here's the brand-named one, for only a dollar more than the Solofill one linked to above!!!
http://www.google.com/aclk?sa=l&ai=C...fillable+k+cup
And if you want it quicker, Walmart has them for just a couple dollars more!
http://www.google.com/aclk?sa=L&ai=C...fillable+k+cup
"Though it seems the secret to NOT getting grounds in your coffee may be to NOT fill the basket too full. However, another reviewer posted the following: Works excellent, although there is a trick to it. You need to take the rubber seal off the needle at the top and flip it over then slide it back on. Otherwise water leaks all over and grounds get in the coffee. If you turn that seal over, you won't have any problems with leaking or grounds getting out."
I haven't tried either of these. I don't have the machine. I've used them at my daughter's house, and I purchased the Keurig re-usable one for her to use. She has voiced NO complaints to me about the filter. My issue with the whole concept is that the coffee, even from the premade cups, is about half the strength I usually make with my press or other methods.