Hi there midhusband. You are right, they definitely should have told you beforehand that you would need a visa. I visited India last year and had sort of the same decision to make as you have to. . I did decide to get the visa as India has always been somewhere I have wanted to visit.
A lot of people get their visa from the gov website: https://indianvisaonline.gov.in/visa/index.html which is a good website and very easy to use. There are also private agencies you can order it from such as: https://onlinevisaindia.co.uk/ or https://india.travisa.com/
Good luck and have an amazing ti