'modprobe' is probably in /sbin, and as a regular user on Fedora the /sbin paths (/usr/sbin, /sbin, /usr/local/sbin, etc) aren't in your $PATH.
When you 'su', do an 'su -' so bash will source root's settings and you won't have a problem.
If that fails, make sure you have 'modutils' package installed.
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JBR