There are lots of options out there so thinking of your needs would be a good start. There's glass options, stainless steel, wood, and plastic (that I know of). There's also lots of places to get these. I have one from Amazon (https://smile.amazon.com/dp/B00W0UFG52/ref=cm_sw_r_cp_apa_fab_5gBGFb8TWPTQG) and one from BentoCo (https://en.bentoandco.com/?gclid=Cj0KCQjwt4X8BRCPARIsABmcnOpXPN37PQnaE1BBhD3HMwFETCvmq37WhKZ36DoVm3B5Po_0oPjF14UaAuWbEALw_wcB).

The Amazon one is pretty good overall and quite large compared to some bentos. I think it's about 1000ml. The bundle I got from bento co I couldn't find but that is their site. They ship from Japan so it can take a while. I personally shop when there are sales because it can be a bit pricey but I really like their stuff (and they have lots of cute accessories😊). The bento I got from there is smaller (to help with portion control), about 700-750ml.

Anything can serve as a bento really. I think the most important things are size (you want enough food) and if your food needs to be heated (from what I know, traditionally bentos aren't meant to be so traditional bentos are not made with that in mind). I hope this ramble was helpful and you have lots of fun with bento.