Hi,
You can buy from Godaddy.com or yahoo or any Toplevel domain vendors.
Just google some thing like " buy Top level domain" and you can get information.
And after that you can configure the domain by setting the domain to point to DNS of X10hosting.
Which is ns1.x10hosting.com
ns2.x10hosting.com
After the domain points to these DNS, you can use a addon domain or parked domain from your cPanel (x10 cPanel) to set that domain to a particular directory.
You can search these forums regarding addon domains and you will find very useful information.
Regs,
VVBB