Hey Straxus, I've seen your Q's elsewhere on this and was wondering if you'd come by... I'm not tooting my own horn much (well, a little) but consider myself something of a plastics afficianado.
I'll help you out here.
First, I suggest stopping by the section on ship building at www.FFURG.com
. Tutorial's sometimes help better than what I can suggest as I'm no teacher, that's for certain. So hopefully I won't confuse you with anything I say here...
First, plastic sites...
Those are, essentially, the best two I can suggest off the top of my head, however I have never ordered plastic sheet from these companies, and instead I've just looked for a GOOD hobby shop in my area that has both of their sales display racks on-hand, so I can actually look/feel/touch and generally get a better idea of the plastic pieces I need.
There's everything from sheets that look like brick/mortar work (varying scales), to deck plating, to corrugated steel and more. And rods, tubes, I-Beams, H-Beams, and the like as well. Sometimes a site won't show you what you need to see, and you need to see it in person. If you have a HObbyTownUSA, I suggest you go there first. Mom & Pop stores are hit-and-miss but one near me carries both companie's and a generally full rack of styrene is available, though certain pieces usually are low or out of stock.
I'd like to ask a quck question too at the start... What are you attempting to build? I can probably help more with more knowledge on what you're going for Straxus, and I'll lend any advice I can.
1) What thickness would be suggested for vehicle frame?
This mostly depends on the vehicle you're talking about. I've worked on vehicles anywhere from 50" or so high to a couple inches off the ground. The larger the vehicle, the more sturdy the frame-work needs to be. There's a certain level of "engineering" that goes into scratch-ship building, but don't let that scare you. I suck at Math, and would never have passed the first semester of an engineering course.
Also the thickness for a vehicle frame depends on what style you go with. Are you going to use "I-Beam" pieces? Are you going to layer up sheets of styrene at key points?
Remember this much. PLanning generally is 50% of making a scratch-built ship. You have to draw things up, do a cardboard prototype helps tremendously, and you'll see weak points you need to sturdy-up and things early on.
2) Best glue for bonding styrene to styrene?
"Glue" in general is not what you want to use... Glue's merely a bonder. Two pieces held together by a third substance more or less. If you're working with styrene (ABS plastics), you want a Plastic/Styrene Welder. I use two types... One's thick, one is very watery.
-Devcon 2-part Plastic Welder (available at Loews, Wal-Mart, etc.). It's a 2-part epoxy but it does fuse ABS together very well.
-Plast-Struct Plastic Welder (Hobby shops only). PS Welder is water-thin, and can take some getting used to working with as it dries quickly. It also will eat right through plastic, so practice with it.
Plastic welders break ABS plastics down so they will "fuse" together and weld like steel would. Two parts essentially become one then. Stay away from super glue, model glue, etc. It's all kid's stuff.
3) Site with affordable sytrene sheets in decent sizes?
As I noted, I like to go to the store for shapes/sizes I need, however if you want sheet plastic you're FAR further ahead to find a distributor in your area. I get giant sheets of varying thicknesses (anything from 1/16" to 1/2") that I need a pick-up to bring home! Yes, a pick-up. I thnk there's a $60 minimum order, but the number of ships you could build is a LOT, or one friggin' huge one.
Find a plastics distributor in your area, you'll be far better off than buying it a sheet at a time. Hobby shop plastic racks usually aren't cheap by any means, and so you'll sink a lotta dough into your models, but if you buy from a wholesale distributor, you'll save a ton and get more plastic than ya know what to do with!
It helps greatly if you have a large working area, or garage.
I hope this helps, and I'll add more later. Gotta jet now.