The Kreg Pocket Hole Jig is often recommended for building fabric frames. So is a Porter Cable US58 Pneumatic Upholstery Stapler.
If you are considering a hobby in woodworking, I would recommend a decent contractor-style table saw (I am partial to my General). But if you just are looking more at being an advanced homeowner/handyman, then there are several great benchtop table saws that can be had for perhaps less money and definitely less floor space.
A good quality cordless driver/drill is almost a necessity. Get one that feels comfortable in your hand, and has decent power to do the tasks you most often perform. Even with a good cordless drill, I recommend getting a corded one, too. Despite what the box says, the battery probably won't last long enough for boring 1" holes with a spade bit through dimensional lumber if you have a good number of holes to drill (e.g., threading conduit through a wall). Plus, if you get a combination twist/hammer drill, you can tackle all of your concrete work with the same drill. But you said you had some drills, so maybe you don't need to think much further about this one.
A miter/chop saw is a must. I have a 10" and it's too small. I constantly wish I had a 12" saw, to handle 6" boards on an angle. Others have recommended a sliding miter saw. I've never used one, but I think I will investigate them fully when my 10" chop saw "accidentally" craps out some day.
I managed for a long time without a drill press and a band saw. Those would fall into the "Nice to Have" category. So would a joiner and planer, unless you are really serious about woodworking, because most places that sell nice lumber will also surface all four sides for you (for a fee, of course). Or, with a table saw, you can buy S2S with a straight line rip. Very convenient.
Think about dust collection in your new workshed. That would be in the "Must Have" category.
I think a circular saw is a must have, too, at least to break down large sheet goods.
A router, to me, is a "Should Have." But if you decide to get a router, then I'd say a router table is a "Must Have."
As for "where" to buy them ... my personal recommendation is to research the tool, decide which brand(s) you are interested in, and then price shop for them. I've bought tools from many of the places big recommended in post #3. Some others are Woodcraft, Toolup.com, and Southern Tool.