I've bought my 20 gal long tanks from Petsmart, they have a setup with a light, bulb, food samples a background....etc. The tanks all have sliding screen tops but are around $100.00 each. The 15 gal tanks i bought from a fish warehouse that stocks tons of different fish and supples and my 40 gal i bought at an exotic animal store. By far they have some of the best prices for tanks and they also have a 70 gal corner tank with a sliding front to it, cool. Take into consideration though that i live in Canada so other than Petsmart only the fish warehouse might also be in the U.S.
In responce to the corner problem of your tank, i have a suggestion. There is wood corner molding that you could use to make a top or bottom, granted i wanted to do that for my 15 gal tanks as i don't have proper tops for them, however i suck at doing mitre cuts. I should remember how to do this as i used to make picture frames but its been a few years and well if you don't use it you lose it.....lol. It was the only time fractions ever made sence to me though, theres alot of math involved. Theres also plastic and metal molding too, the metal stuff we used when putting up a few walls after drywalling. They protect the corners and is put up before plastering.
I have a 25 gallon tall tank that i wanted to use for Zeek (Petsmart again) but the sliding top opens the entire top of the tank and well Zeek is a smarty....he's good as trying to escape. So i thought about removing one side, what would be the front and installing sliding glass doors. The track molding my dad found has really large spaces, for really thick glass so i'll probably go with plexi glass instead for less weight. All i need to do now is get that front piece of glass off without breaking the rest of the tank 
Red 
>>Where do you order your tanks from?
>>-----
-----
Crested Gecko Zeek:1.0
LEOS: Boo: 1.0 normal , Bronx & Nala: 1.1 blizzard,
Lily: 0.1 patternless, Abby: 0.1 albino, Zoe: 0.1 reduced pattern, Dot: 0.1 hypo
Chip: 1.0 papillion (small dog)