Where can I find BCA textbook guides

BCA course syllabus and lessons will be sent by the university and the notes sent by them will be good. I don't think you require any other guides separately.
Annamalai Univerity is very famous for distance education and I know many people who did courses through this institute and obtained the degrees. If you read the notes given by them, you will understand the subjects well. They will mention some reference books and if possible you can refer to those books.
If you have any friends who are doing BCA in the college, you can ask them their notes and see them. As we go for higher classes, depending on the guides is not a good practice and you will not get a grip on the subject properly. Even though you get the degree afterwards getting a job will become very difficult. So I suggest you follow the notes given by the college, Still, if you feel you want guides you can go to a good book shop which sells college textbooks and guides in Trivandrum.

It is really a good news that you have registered yourself with Annamalai University for the course of BCA since this university imparts quality education in almost all the courses including BCA through the distance mode.
Regarding the syllabus and the course structure to be followed in each semester of the course, you will get their instructions and the same is to be followed up. There are plenty of books in the market providing you help for the upcoming semesters but the same might not cover the syllabus of your course since the contents are written in general for all aspirants interested to complete BCA. Omission of any chapter/ contents can impact your smooth preparation for the ensuing examination.
Hence it is better to follow up the directives of the authorities of the concerned university for your better scores.

It is good to search for such guide books from the booksellers of Old/used books in your area or you can approach the following old/new book sellers.
1.sukumar Book Stall, MG Road over bridge junction,MG Road, Trivandrum
2.Global books,15,Police Camp Rd, Nandavanam, Palayam, Trivandrum
3.Book Land,Palayam, Trivandrum
4.Alpha Book Center,Kallambalam, Trivandrum

You have registered for distance education and you would be getting the standard course material from the institute from time to time. It is advisable to make your own notes after studying that material. Making one's own notes helps in bringing the original touch in it and that helps in scoring in the exams.

Anyway, for additional material, one might require some standard textbooks and guide material for which one has to search in the leading textbook stores. I have also found some information in this regard and would like to share it here.
1. For Web designing - Web Coding & Development All - in - One For Dummies by Paul McFedries; Web Designing and Publishing by Prof. Satish Jain; Mastering HTML, CSS & Javascript Web Publishing by Laura Lemay.
2. For Scientific Computing - Elements of Computer Oriented Numerical Methods For B.C.A
by M. L. Bhargava; Elements of Mathematics For B.C.A by Dr. Dalip Kumar.
3. For prose and composition - English Grammar and Composition by Rajendra Pal; English Grammar and Writing Skills by Sudhir S Padhye.

Thank you all for helping me out, much appreciated.

Annamalai University is a reputed university where you have registered yourself. It provides distance education. There is a rule of every distance university that they send syllabus and course material after registering the candidate. So, you need to be worried about this. You should try to make notes yourself with the help of course material. It is the best way to command over any subject. If you seek more help then ask in friend circle about those who have already completed BCA. Moreover, you can search the internet where you will get many online guides related to your subject. You can also search guide in a local bookshop.

There are many books available that can help you to understand the whole syllabus and related subjects. Many languages have easy and good explanation language, that will help to understand them. But before buying any book have deep look at its content and reviews. There are many more books available on online websites, you can also go with online videos that help you to clear your concept and make your basics also stronger. Here are some books and guides for your specific subjects.
1. online systems and web designing - There are some books for web designing -
Web Designing and Publishing by Prof. Satish Jain and M. Geetha Iyer
Web Designing And Development: Training Guide
Learning Web Design: A Beginner's Guide by Jennifer Robbins
2. Scientific computing -
Fundamentals of Scientific Computing by Bertil Gustafsson.
Elements of Scientific Computing
3. English prose and composition -
English Prose Composition: With Illustrative Examples by Edward Fulton (Author)