Portugal Road Tolls

Hi all

We have over 30 years experience towing in Europe and have usually cracked the motorway toll systems. Researching our imminent first trip to Portugal, it all seems rather haphazard in that country
I know there are two types of toll: standard type with toll booths that have the usual payment methods; then there are the electronic tolls - no booths but all paid for with a device or pre-pay card.
It seems to me the best solution is the Via Verde system where you hire a gadget for your stay - €27 returnable deposit, €6 hire first week, €1.50 subsequent weeks. This will cover all toll road types.
Looking at the Via Verde website and others, it states that these can be bought at Via Verde offices and some service stations. Nowhere does it state where these might be. Apparently you can buy online but when you hit the relevant page you are faced with a logon request - with no way of signing-up. Seems risky to travel in the hope you MIGHT find somewhere to buy it.
Has anybody experience of this in Portugal or any other method of payment?