ISP is inserting ads into web pages
There are three ways to beat this:
Only go to the https: version of the website. There are browser add-ons that help you do this (e.g. HTTPS EveryWhere). This won't help you if the sites only have a http: version.
Use a Virtual Private Network. There are many VPN providers available, probably some free ones as well.
Install an Ad blocker (like AdBlock Plus). Chances are that the IP adresses that the adverts come from are already in the block list of the add-on. If they are not, you have to trace the domain names or IP ranges that they come from (they may very well come from one ad provider), and manually add that information to the block list. Data from those web addresses will not be retrieved.
Methods 1 and 2 ensure that your traffic is encrypted when it passes through the ISPs' computers. They cannot see (and change) the HTML/Javascript that way.*
Although you say I can't change my ISP, that would be the preferred solution. Leave and tell them you left for that reason. Losing customers is the only argument they will listen to, and this will help other customers too.
If you are absolutely sure that you ISP is doing this, I invite you to edit your question and name them.
* This assumes that your provider has not asked you to install their certificate so that they can decrypt your traffic. This is unlikely, I have not heard from providers that do so. If they do, method 1 is not going to work. A VPN still will.