A quick makeshift replacement is one of those little plastic tubes you find inside spray bottles. I have one tucked away for my PRS-650, just in case. It fits inside the stylus "compartment" (though it would fall out if I tipped it, so would need to find some way of preventing that), but in testing, it works well, and it certainly won't break. If I wanted some stiffening, I could insert a thick wire (I have a coil of a perfect fit in my tool cabinet), but I don't use the stylus for drawing, so I certainly couldn't speak to how it would work for that.
I've seen the ridiculously expensive replacements and would simply refuse to pay that much. In other threads, people have mentioned other things that work also, such as a cotton-tipped swab, etc.
UPDATE: See
this thread for info on ordering an inexpensive replacement from a fellow MR member. It's a slightly bendable plastic replica of the original that fits perfectly into the stylus slot. I ordered one and am very pleased with it. Read further posts below for verification that it fits the 350, 650, and 950.