Supply is dying out in a lot of schools, I know, because I am a secondary trained teacher who had a career break to raise kids, and I was looking for experience to get me back to the classroom. Agencies will always sign you up, but work is sometimes scarce, partly because HLTA's are used as in-house cover. Plus, each agancy requires a separate CRB check, which mounts up. Some LEA's only use a certain agancy, some schools only use a certain agancy, very few take you on directly. EPM is a big one.
Most of my offers came from the neighbouring urban areas, mostly in sink schools and so on that struggled to retain staff. I found most offers were cover supervisor, or roaming cover, not subject specific. It really didn't suit me as I wanted set days, not early morning calls, due to sorting out childcare and so on. In the end, I opted for a HLTA post to ease myself back in and it's fitted me a treat thus far, no work to bring home and so on, and plenty of opportunity to gain experience again. Plus, it's helping me swap from secondary to primary, which is what I wanted, no longer having the energy to deal with roomfuls of teenagers and coursework nightmares again! That could be an option for you too, 2 of the teachers in my current primary are ex-middle school teachers.