Look on Amazon for the Eagle Waistband amplifer. You should be able to get one for around £25. They are OK, but not particularly sophisticated!
Has your speech therapist offered you LSVT therapy for your voice? If they don't offer it at your local NHS clinic you could look for a local private or NHS clinician on lsvtglobal.com
I provide this therapy in Bristol. It is intensive. 16 hours of therapy over 4 weeks with daily home practice, but has good results.