Failed SSD hard disk

I need some help for my college age sons computer.  6/15/15 I bought a pair of Dell inspiron 11 computers. one for me one for my son.  In a moment of insanity I forgot to get an extended warranty on his computer.  I have been very satisfied with my computer in every respect except the keyboard.  My sons computer had a solid state hard drive fail in the 1st 3 months which was covered under warranty.  He is now out of warranty and the hard drive has failed again.  He took it to the computer dept and the college and they confirmed the hard drive has failed.  I want to purchase a new hard dirve and replace it myself, I have moderate tech expertise with computers and it doesn't look hard to do.  My question is what replacement part do I need to order.  I want to make sure whatever I order fits, is reliable.  Im not sure what website anyone would recommend I order from.  The failed drive is 256GB, the replacement could be the same size or slightly smaller in storage capacity.  Fortunately he did not lose any data but I don't want to keep purchasing SSDs.  I thought they were supposed to be more robust.