I am thinking of getting myself a cushion/pad for kneeling. Currently using my pillows but they aren't very thick. I've briefly looked online but there are so many options I am not sure what to get. Any recommendations?

If you're not disabled and under 50, kneeling on the hard floor is a great sacrifice and act of mortification.  Much less painful than what the saints practiced.

In my case, the people doing penance will be those who pick me off the floor!

Welcome to the club.

Whatever works for you ... don't trust the generalizations of some anonymous people who don't know you.  If you have problems with your knees, such as old sports injuries, or arthritic knees, etc. ... it would be imprudent to kneel on "bare floors" or even the un-padded wooden kneelers.  In fact, even if your knees are in good shape, kneeling on hard surfaces can cause damage over time.  Other things can be done for penance that don't involve potentially damaging your joints.  Our Lady instructed the children to stop certain penances they had undertaken since it could have caused them physicla harm.

So the answer to your question is entirely subjective, analogous to how some people sleep better on softer mattresses, but then other people hurt their backs on soft mattresses.  It all depends on your physiology, how you tend to position your body when you sleep (sides, back), and whether you have other issues (back or knee pain etc).  Again, you could sanctimoniously declare "just accept the penance", but then if you're not getting enough sleep, that can cause emotional, psychological, and even spiritual issues (+Lefebvre insisted on seminarians and priests getting sufficient sleep ... and I'm sure he knew cases where some had "cracked"), or if you don't sleep on the right surface, you might wake up and then barely be able to walk or function or do your job (especially if you do manual labor).

This is why Prudence is considered the Queen of the Virtues, where you have to weight and take into consideratin many different factors.  No, doing as much penance as possible in all areas is not some absolute good, and savors of that leaven of the Pharisees.  Instead, sometimes various penances can do more harm than good, or are done for the wrong motives, e.g. pride, something that offends God more and keeling on a cushion or even just sitting down.  That's yet another reason why it's important to obtain good spiritual direction if possible.  Otherwise, use sound judgment while not turning things that are relative, ends to various means, as if they were ends in and of themselves.

By orders of magnitude the greatest moritifcation should be of one's pride, and doing things only under obedience to a director helps offset that.  People aren't given to pride, for instance, when they abstain from meat on Friday because they're doing it out of obedience to the Church's law.  In this, for instance, if you were to opt for some thick cushions, you could humble yourself by declaring yourself inferior to those who can keen without them, and that would please God much more than someone thinking highly of himself for kneeling on a bare floor.

How about the pads they sell specifically for kneeling outdoors while gardening?  Try an outdoor supply or gardening shop.

We got a bunch of these garden kneeled pads for our wedding since it was in a gym with hardwood flooring.  Later we donated then to a mission chapel.  🥰

  It's not about the discomfort.  If you mess up your knees and, oh, say, work a manual job and suddenly have a hard time doing it and lose the ability to support your family ... or same thing goes for your back.  That's actually a significant drawback with kneeling on the floor, instead of on a slightly elevated surface, since depending on what you have to do with your feet, it could end up being an ergonomically challenging position.  I've known people that had to go on disability as they got older since they could no longer work due to physical issues.  I'm sure it pleases God to engage in imprudent activity just so you could act like some "big shot" for "eternity".   I said quite clearly that you can certainly find other ways to accept "discomfort", but none of these are absolutes.  Turning things like this into absolutes is textbook "leaven of the Pharisees".  That's precisely what they did, turn various laws, rules, and regulations into ends in themselves rather than as means of serving God.

 I'm sure it pleases God to engage in imprudent activity just so you could act like some "big shot" for "eternity".  I said quite clearly that you can certainly find other ways to accept "discomfort", but none of these are absolutes.  

Humility is not being a "big shot".
Kneeling is probably the least discomfort to offer up.