If you have a Home Hardware near you, they have a house brand that is a little cheaper than the GE I and is aquarium safe (around $4 or 3 pack for $10). If you are going to rebuild, do get a caulking tube as it will be much easier.
I built my first tank when I was 15 and it is still in use 44 years later, still held together by the original silicone. I used a squeeze tube, but wouldn't recommend it.
Join a fish club. Don't have one in your area? Start one.