I use Verizon on iPhone here but in last Europe trip sucked up the very limited data on one of their international plans during a month long trip - within the first couple of days ! using uber app. Expect to use uber a bit in Lisbon, Porto and Barcelona. Can I get a SIM card here that will work there- maybe for one of my older iPhones..... or do I need to get one upon arrival in Lisbon..... suggestions for where?
Either buy a SIM when you arrive in Portugal and use it in Spain too...or perhaps buy one ahead of time from one of the countries that doesn't require you to register it with a passport in country - e.g. from the UK or the Netherlands. I use a Dutch Vodafone SIM bought on eBay last year. Used it in Portugal earlier this year without issue.
This is mostly still valid from last year - except you get 2X more data now, and credit expires 6 months after you add it unless you have new activity on the SIM, add more credit, etc.
I don’t have info on buying one before you come, but know it’s very easy to purchase a SIM once you arrive. MEO and NOS stores are plentiful all over the area. You can purchase a SIM good for one month for about 10€. Depending on how much data you use, it may be enough for the entire month.