OK first thing is the shipper is not required by law to give you placards, if they don't have them, then you have to note it on your paper work and get them. You as the driver have to ensure that the proper placard is used, which means you have to get it if they don't have it or refuse the load. I was loaded all ready to go on one load and they didn't have all the placards so I had to have my truck unloaded, drive to the nearest truckstop (60 miles away) and buy the right one. This has happened to me a few times and I always ALWAYS ask for three extra when I get them whether I need them or not.
Second, the flip type are the best to get, I doubt you are going to be hauling anything special that requires a special panel so a standard one is good enough, I would also add a slide in holder type next to the flip type just in case you haul two hazmat items. You don't want to tape the placard to the truck because some DOT officers get all bent over the tape thing crossing the lines and if you use the sticky ones, then they may screw up your paint or even peel it off.
You will pay about $35 to $45 for each flip type and the slide in type will run under $20 (this is not from the link below, they are cheaper from the link), money should not be a factor because if you ever get a hazmat related fine, it is cheaper to spend $200 on placard holders than the $700 for the fine.
Where to get them?
A good truck dealer will have them, trailer repair and supply places have them too. I would see them before buying them, I bought a bunch online once and was p*ssed that they sent me three different types with different hazmat placards on them.
If you want really good ones;
Placarding Systems from Labelmaster
LabelMaster will sell you a 10 panel style for about $27 each which is a good price. If you do a lot of Hazmat like I do, consider getting the 17 panel one. The slide in types are under $10 each.
OH and one last thing, the little clips or snap levers on them sometimes pop off, you can put them back on with a small flat screwdriver. Also after a while you may lose a couple, Speedway motors and the above link were the two places I could find them at a reasonable price. AND before I forget, sometimes the rivets will fatigue and pop off, just get pop rivets (steel) and use them to fix it.