Q: Can I buy installation disks from the Apple Store?

No.

But you can contact Apple Store Customer Service at 1-800-676-2775 and ask if you can purchase an install disc for your Mac.

Have the Serial# handy so they can see if they have te exact replacement Disc(s) available...

How to find the serial number of your Apple hardware product...

http://support.apple.com/kb/HT1349

Just to expand on previous replies...

If you are very lucky Apple may still have a set of original installer discs for the model computer you have (about which you are not very specific).  That will be for whatever OS came with the computer originally which may well not have been Tiger.

It is extremely unlikely Apple will have retail version Tiger discs.  I guess there is no harm in asking but I recall people asking this 5 years ago and they said Apple was sold out. Still, maybe they find something in the bottom of a box somewhere.  You will definitely for sure NOT find this in a brick and mortar store. You have to call Apple, and maybe customer service rather than sales (I think they only know what they see on their computer).

Failing those you will have to try places such as e-bay, Craigslist, etc.