I think there are online retailers but nobody I have seen specifically recommended. There's the usual used parts sources such as e-bay and Craigslist but make sure you get a retail version disc. Some people try to re-sell the gray colored discs that came with a computer but those are designed to only work on that particular model. The retail Tiger discs are black. There are also limited discs that are designed to only work when installing on top of Panther. That would be okay in your situation except if you should ever need to re-install you would have to install Panther first again.
See other recent posts in this forum about upgrading to Leopard. Both Tiger and Leopard are no longer supported by Apple. You can run newer versions of things such as iTunes on Leopard but it will still be several versions out of date. With Leopard you can no longer run OS9 generation applications in Classic. Leopard will also make more demands on your computer. Check the specs for minimum system requirements. I think your 512 MB RAM meets minimum but when Apple says minimum they mean it and some people complained about Leopard running slowly. I run Tiger with 2 GB RAM.
I have a copy of Leopard but I have never bothered installing it. Yes, there are a few applications I could run in it that I can't with Tiger, but I am generally pretty happy with Tiger. You need to review your exact needs but I suspect I probably won't really bother with Leopard and my next jump will be new hardware and something that will still run 10.6 and PPC generation applications.
As for cost, it is what the market will bear. I haven't checked prices recently but for quite a while Tiger and Leopard held strong on resale. I can't imagine it going on for much longer because even if people are happy running old Macs the hardware will eventually give out and people will have to buy newer computers.