If your not looking for anything special. I would go the last day of the show. You can usually bargain with the seller. Since most of the stuff they are selling they are trying to get rid off to make more room.
If you see one you like but doesnt look that healthy dont buy it. You probably...
Just have a little dish with rep cal calcium without d/3 in the tank at all times. Your geckos will actually will go to it and lick some off it.
As for the vitamins and calcium with d/3 just put some in your food dish with the meal worms.
Its good for vitamins but I wouldnt use it every time for gut loading. Its better if you go with rep cal calcium without d/3 in a small dish in your tank. And dust with miner-all every other feeding.
Actually instead of building, you can get use aquariums for really cheap now. Check craiglist or recycler if you have one. I have seen 40 gallon breeders for $20. Good luck