Wanting a wider look for my 96 tracker, so looking to go with 7 or 8 inch width wheels when I go to new 235/75/15's. Question is,do I have to buy new wheel for my spare? Or is it ok to run the stock wheel with my old 215/75, for a bit if I get a flat?

As long as you haven't got a spool or locker in the rear axle, you'd be ok putting a different height tire on the rear.  You could run a different height tire on the front too, as long as the front doesn't have a locker/spool, or if it does, as long as you don't run it in 4wd or with the hubs locked in.
The wheel size won't really make any difference in any aspect, it is all about tire height.

Whilst Sky is 100% correct, if you're going to run a different size spare, and that includes "space saver spares" aka "donuts", you should be aware that running with different height tires on the drive axle for long periods is detrimental to the health of your differential.

So if your spare has a different height to the rest of your tires, use it as a spare - and keep the usage to the minimum required.

Personally - I run the same size tires on all five wheels on my four wheel drives - I do have a sport hatch which has alloy rims fitted with wide low profile tires and the OEM sized steel wheel & tire as a spare, but the wheels & tires there were very carefully selected to maintain the tire height (and gearing).

Whilst Sky is 100% correct, if you're going to run a different size spare, and that includes "space saver spares" aka "donuts", you should be aware that running with different height tires on the drive axle for long periods is detrimental to the health of your differential.

So if your spare has a different height to the rest of your tires, use it as a spare - and keep the usage to the minimum required.

Personally - I run the same size tires on all five wheels on my four wheel drives - I do have a sport hatch which has alloy rims fitted with wide low profile tires and the OEM sized steel wheel & tire as a spare, but the wheels & tires there were very carefully selected to maintain the tire height (and gearing).

Right, I don't recommend running a different sized spare for extended periods of time.  Just long enough to get to where you can get the original tire repaired/replaced.

Much appreciated guys