Have noticed these have disappeared from store shelves around me. Are they still being produced?

I had also been buying some ge branded 4' lamps advertised at 2900 lumens.

What are my alternatives for 4' and 8' lamps with good lumen output?

It wouldn't surprise me if the were discontinued after Versailles closed.  If somebody comes across some from the Philips Salina plant, then they might still be in production.  The GEs might be your best bet as far as readily available lamps go.  Some T10s such as this Satco lamp are available and very bright (over 3500 lumens). 

The ge's are great for my 4' fixtures and the T10's would be also.

I'm really more concerned about the 8' f96t12's. I had been intending to put about 10 8' fixtures in my garage but I'm having second thoughts if I can't get good lamps.

the Phillips CW supreme F96T12s are a good choice, sure they're not the highest lumens (5000) but the CRI fools the eye into thinking they are brighter, or even the GE version which is Phillips made minus the green endcaps