If you're a geography data nut and a cheapskate like me, then you owe it to yourself to bookmark the Free Geography Tools blog. In an especially relevant, recent post, Leszek Pawlowicz explains how to make your own Topo maps for a Garmin GPS. Rather than copy and paste the entire post here, I suggest you click on this link to read it for yourself.
Leszek notes that Garmin is beginning to make the 1:24,000 scale US topo maps available on microSD cards, for those units that can utilize them. Unfortunately, the Garmin microSD cards sell for around $100 a pop. Luckily, Leszek goes on to describe other, cheaper options for getting the topo maps you need, with plenty of links to other GPS Topo data resources. There's also a link to a tutorial site that explains how you can make your own Garmin Topo maps.
Thanks for the great information, Leszek!