Aria here, one of the Social Care gurus at Shopify.
You're correct in saying our banking partners don't allow us to process cannabis sales using Shopify Payments. You can certainly sell cannabis on our platform (assuming you are only shipping it to locations where it's legal) using a third-party gateway. You can find the full list of available gateways in this guide. I would recommend you take a look at Authorize.net but you may find others on the list that work for you. You'd set up an account with your chosen gateway first, using their website then you can complete the setup in your Shopify admin by going to Settings > Payments.
Let me know how you get on with that.
There's actually quite a lot of credit card processors on the list I provided but most of them won't allow the sale of cannabis. You could also try eMerchantBroker (EMB).