I've been tying for about 1.5 years now. When I started I just bought whatever bait hooks were available local. Thousands of them! Someone on another forum seen my flies and sent me standard Mustad dry fly hooks. Let's just say I won't be returning to my old ways any time soon.
Do other hooks work? Yes.
Do they work as well? No.
Hook up rates on fish are much higher on dry fly hooks. I also found hooks with barbs on the shank case your flies to be short lived. Another difference is standard fly tying hooks do not have a curved shank like many of the other hooks on the market.
If money is an issue you can do one of the following:
Use wet fly hooks which can be as much as half the cost. You can use them for foam flies and wet flies.
Buy bulk mail order.
Apply for a Cabelas (or other) credit card that awards points. Use this card to buy your gas, pay your bills, buy lunch, or other things us use cash for now. Pay the card off each month. Use the points to order hooks for free.
I know people that make their house payment this way too. In one years time they can get free bows (archery equipment), new fly rods, wadders, or all the tying supplies one could care for.
Bottom line is the profile of the fly tying hooks make better looking flies that catch more fish. The points are normally much sharper too. Their free so why not?
I tie most of my gill flies on size 14 hooks, although any size a fish can get in its mouth will work. If you start with a 6 and add wings legs, or whatever sometimes the fish bump them with their lip and the fly doesn't get deep enough for a hook up. if I'm catching a bunch of small gills I'll go up a size to cull the smaller fish. I continue this process until I only catch the size I'm after.
Just in case you don't know it, foam spiders are far from the best gill flies. They work but I can catch 10 or 15 to every 1 caught on a spider in my area.
I'll see if I can find some pictures for you.