Showing a pic of your particular stairway would be very helpful. As a painter of 35 years, I'm pretty sure I've tried them all, but I can't give you best suggestions without knowing your particular layout. As far as ladders, box stores will actually have a decent selection of quality offerings. For levelers, I'd avoid any that look like the levelers in this 1st pic since they require having to pick up the entire ladder and then place it back down as level as possible in order for it to level "automatically". They become increasingly cumbersome with larger ladders. I've all but phased these out on most of my vans, but still have a few stragglers. Nobody likes them.
Instead, I'd suggest looking for ones with the springs which can be adjusted individually. They have the ability to be exponentially more adjustable than the ones in the 1st pic. I've got about 14 pairs of these and some I've had for a decade or more. Very safe, reliable, and easy to use. Here's an example:
Depending upon your stairway layout, you may or may not need an additional piece of gear if your stairs are curved or if you're having to stretch pretty far out from the handrail. In those cases, I'll use a Pivit tool on the low side of the ladder. They're basically just a big triangle wedge with grippy rubber on the bottom and a skateboard grip tape texture on the top to set your ladder. They can be inverted to use the opposite angle, or they can act as a ladder jack if you had 2 to secure a plank, (but that's only advisable for more experienced users). Just trying to explain that it has many uses other than just a wedge. Here's what they look like...
