Really depends how much you want to spend, Anker is a really good choice. I have a powercore 20100mah. I've owned it since October 2016, and it has been great, never given me any problems. You can get this one. It has PD, and comes with a high watt charger brick.
https://www.amazon.com/dp/B07XRJZXKY/ref=cm_sw_r_wa_apa_i_ZlU3EbRRFK9X5
Anker PowerCore+ 26800mAh PD 45W with 60W PD Charger, Power Delivery Portable Charger Bundle
I have an Anker PowerCore+ 26800mAh PD which delivers up to 45W PD, which as far as I know should be just as good for juicing the Steam Deck as the actual charger.
https://www.amazon.com/Anker-Charger-PowerCore-Portable-Delivery/dp/B07XRJZXKY
Bit pricey, but well worth it. And the capacity will definitely top it up several times over. The Steam Deck has about a 5300 mAh battery if memory serves; even accounting for some efficiency losses from the power bank to the device, it should still charge the Deck at least four (possibly five) times over before it itself needs recharging. That'll give you a pretty solid amount of portable playtime. Possibly more importantly, it's still considered airplane-safe as part of your carry-on luggage, which can restrict anything over 27,000 mAh.
SD Cards, obviously as big as you're comfortable with. Just make sure they're UHS-1 for maximum speed. Any reputable brand will do, just make sure you get it from a proper seller on Amazon (preferably from Amazon themselves) and not Fleabay, and pay attention to the prices - if it seems too good to be true, and similar ones aren't fairly close to it in price, then it's probably a scam.
This 512 GB one will do for a lot of people and won't break the bank:
https://www.amazon.com/Lexar-High-Performance-512GB-microSDXC-UHS-I/dp/B07KBQQ82D/
Hopefully in a few weeks I'll have my email too. At the rate that they've been pushing that queue, I might have my email in a week (I'd be likely one of the very last ones in that batch assuming they can push about 1h7m or so each of the next few ship dates), or failing that, next Thursday at latest.
Just in time for my birthday. :)