Your question (and the comment discussion) basically boils down to "I have a website where users upload stuff. I want to be able to store more stuff, but my ISP's quota is too low to allow that. What do I do?"
The (right) answer is simple: Buy more disk space (your ISP will almost certainly be willing to sell it to you!) or move the site to a provider who will give you adequate space.
If you want to go with hackery you can easily set up space on Amazon EC2 or similar services or write up some custom store-and-retrieve scripts and use your home machine (or something like rsync.net) to provide back-end storage, but this always risks a disruption between the back-end storage and the front-end web server, and as tombull89 pointed out many home ISPs have a "no servers" clause (using your system as back-end storage could be construed as violating such a clause, in which case you can kiss your internet connection goodbye...)
You're almost always going to be better off using a hosting service that is capable of meeting your needs rather than trying to home-grow a solution.