It doesn't matter whether you are starting out or more experienced. Have an agent or not. You need to self-submit on roles. Here's a visual of all the best places before we break them down.

 CASTING RESOURCES WORLDWIDE
Makes sure you follow the links and investigate, before you decide which are the best platforms for you. 
1)  ACTORS ACCESS https://actorsaccess.com/
2)  CASTING NETWORKS https://www.castingnetworks.com/
3)  800 CASTING https://800casting.com/index.html
4)  BACKSTAGE https://www.backstage.com/
5)  CASTING WORKBOOK https://home.castingworkbook.com/
6)  CASTING FRONTIER https://castingfrontier.com/ 
7)  SPOTLIGHT https://www.spotlight.com/
8)  STAR NOW https://www.starnow.com/
9)  CASTING NOW https://www.castingnow.co.uk/
10)  enCAST https://encast.eu/
11)  MANDY https://www.mandy.com/
12)  IMDBPRO JOBS LISTINGS https://pro.imdb.com/ 
13)  FACEBOOK GROUPS FOR ACTORS https://www.facebook.com/
14)  LISTINGS AT FILMS SCHOOLS/UNIVERSITIES/COLLEGES 
15)  CAST IT TALENT https://www.castittalent.com/ 
16)  ARTS COUNCIL JOBS SITE https://www.artsjobs.org.uk/
17)  THE STAGE https://www.thestage.co.uk/jobs/
18)  EQUITY https://www.equity.org.uk/ 
19)  CASTING COLLECTIVE https://www.castingcollective.co.uk/ 
20)  CASTING DIRECTORS OWN SOCIAL MEDIA ACCOUNTS 
21)  SEARCH THE TERM 'CASTING'/'CASTING CALL'/'AUDITION'/'SELFTAPE' IN THE SEARCH BAR ON FACEBOOK/TWITTER/INSTAGRAM. 

So there you have it. 21 different places for you to self-submit on your own work.

I hope you find this incredibly useful.