What can be the problem if I've changed the Alternator, tested the battery and Starter (both "good"), have tried jumping the car twice, yet still just getting a "click" when I turn the key??

You missed the basics. Check battery terminals for corrosion and make sure connections are tight

I did have one terminal that had a little corrosion, cleaned that up and even filed the inside of the terminals a bit (per suggestion from my old-school dad), tightened them up, and they appear to have a good connection... . Despite the battery testing good I tried a new one (upon suggesting ) and still getting the same thing! ??

It had to be a bad battery cable. Double check the one that was dirty and make sure the cable itself is not corroded, U'll see like grey dust coming off the cable, it it is, replace it. Another way to find out, try putting a jumper cable directly from battery to starter along with the existing cable that u cleaned and see if it cranks, if it does, replace cable. If it still doesnt crank do the same thing on ground cable. If still doesn't start, check starter

Am now told by mechanic (took car to shop - NEED a working car) that Starter and battery ARE bad ... So, I will be towing car back home and repairing myself, can you tell me a good affordable entitiy that sells Starters ?? Thanks, appreciate the help

I buy my batrerues at Autozone, it's cheaper and it's convenient in case u get stranded aywhere, u can always get an exchange at any store nation wide

U can buy after market starters but don't recommend them, I've had issues with them and alternators. Autozone has them too