Eleanor - is this for stage or screen?
It is very difficult to recommend monologues for someone we don't know. It needs to be something YOU can connect to.
There aren't many short cuts to finding good monologues - but here are some ideas that might help:
1. Look for playwrights local to you - or local to someone with your accent - this can provide great hunting ground.
2. Look in books of monologues - find one you like and then look for other plays by the same playwright.
3. Ask people who know you for recommendations - members of amateur dramatic societies are often a good source of ideas and may have play scripts you can look through.
4. Look at sites like doolee and stagescripts. You can often find out about the ages of characters in the plays from the synopses.
5. Look at plays written for school and colleges (stagescripts has a selection) as they may have characters the right age. You can download bits of the script for nothing.
6. If you are in London then the National Theatre bookshop and French's have fantastic selections of plays and they are used to people browsing through them for ideas!
7. Go to the theatre as much as you can - look at what's on and select plays you think might have appropriate characters in them from the synopses.
8. Attend new writing festivals and arts/fringe festivals - often you can volunteer to help at these and then get free entry to lots of events when you are not on duty.
9. It doesn't have to be a speech all in one chunk - you can cut and paste bits together providing it still makes sense.
One play (and playwright) I have often recommended is Henna Night by Amy Rosenthal - there are two feisty female characters in it with several very good speeches. However, it may be overdone these days.
Good luck!