Where can I purchase filled ink cartriges for my HP6500?

Read on and see if any applies to you.
Check that you removed the protective tape. some cartridges have two of these pieces of tape one over the head and possibly on on the contacts to the printer.

If you recently re-filled or had the cartridge refilled or even bought one of the many cheaper replacement cartridges you should read on

Most modern cartridges have an ink level chip built on to them.
Should the level drop below 25% then most of these chips lock up and will not reset.

Re-filling them will have no effect

If you are lucky you should be able to find either a chip resetter around on the INTERNET or maybe even local

OR

Even better empty cartridges that never need to be reset.
The second option is my preference

I realise these are not for your specific printer but there are others listed on both those sites as well.

cheaper cartridge problems have been on the rise recently and that is for ALL makes of printer.
If you try to clean the contacts be VERY careful cleaning contacts and ONLY use isopropynol on a lint free bud or cloth.
Also be very gentle.

I have seen the contacts cleaned and cartridges ruined with other methods of cleaning.
Rubbing too hard can also cause a badly set chip to move making it inoprable.

I dont like full prices replacements either which is why I always provide the links
I usually reccomend that people take back any cartridge that does not work right from the get-go.

I dont know of anyone who has had a problem doing that be it a shop refill or otherwise.

No matter if I paid $1 or $100 for a cartrdige I would still expect it to work

Aftermarket cart are nt preffered.. you should purchase the genuine hp ink cart ..