One material that might suit for testing anywhere from room temperature through 500C would be a variety of 304 stainless (there are several varieties within the "304" designation). 304 is not subject to phase changes at 500C, and it is the most readily available of the stainless grades.
If you will cut specimens from plate, then 304L would be appropriate.
Bar has a slightly different chemistry to maximize machinability. I believe just plain 304 would suit.
You might want to avoid 303 bar, the high sulfur free machining grade, to have slightly more uniform properties.
Whatever metal you pick, if your aim is to check uniformity of testing machines you must have uniform metal. The properties of a given alloy vary from heat lot to heat lot, from bar to plate, and from one producer to another. For that matter, they vary from one plate to another even if it is the same heat lot (identical chemistry). Rolled Alloys is one distributor that can get you metal all from one individual plate, if you wish.
Get yourself a large quantity of your chosen metal from one mill source, certified to be all the same heat lot. Usually one of the metal distributors can help.
If you machine specimens out of plate, be aware that the properties in the mill rolling direction ("with the grain") may be slightly different than transverse to the rolling direction ("across the grain")
If you have further questions, I'll be glad to address them. I log on maybe once a week or so.