Going from Tiger 10.4 to Leopard 10.5 is not an update, but an upgrade.
You can purchase Leopard from Amazon > http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=1 0.5+Leopard
Before purchasing, make sure your Mac meets Leopard's requirements > Requirements for Mac OS X v10.5
I'm trying to update my OS from 10.4 to 10.5
10.4 and 10.5 are both no longer supported by Apple, no security updates.
With a 10.4 era machine, my advice is to stay on 10.4 until the wheels fall off, reinstall 10.4 if you have too from the disk after backing up files off the machine.
Don't bother even upgrading the OS because, 10.6 is going to be ignored this summer when 10.8 is released.
Apple has a terrible history of only supporting the last two OS X versions in circulation.
10.6 is as far as you can go with that 10.4 machine, so why bother spending money for upgrading and software for a machine that's just going to be ignored for security updates again?
Stay on 10.4 and be happy you really got some good life out of that machine, don't use it for online banking etc.
Save your money for a new 10.8 machine coming out after this summer and be prepared for a radical change.
Note: 10.4 is actually less of a malware target than 10.5, very few people on 10.4 still