Hmm, I was looking at the Hagerty valuation site:
https://www.hagerty.com/apps/valuationtools/search/Auto/1958/Pontiac It looks like the Bonneville is the star of the show, and any convertible will be kind of spendy. I then popped over to Mecum's site to see how many had been offered at their auction, and it was something like 27 over the last few years. Then I looked into Hemmings, there were something like 60 Pontiacs from the 1951- 1959 model years, as opposed to, say, 573 Fords for example. So, it looks like you're looking for a somewhat uncommon car, and you may be in for a bit of a search.
Now, are you sure you want a '58 Pontiac, and only a '58 Pontiac? There are lots of other interesting cars in that vintage that can be had for not too much money. One thing I've noticed is that it's pretty common for someone to buy a car, keep it for a few years, then sell it and get something different. Maybe something else will catch your eye, you've got to go out and see them in person. Mecum has an auction in Houston the middle of next month, and you've got an airplane that can get you there quickly. Admission is $30, and there should be at least 1000 cars there.
I was born in 1957, but if I were looking for a classic car it would be from the late 30's, for some reason they just appeal to me. If you look around, you may find something that you like even better. If you don't, well, that just gives you more motivation to look.