How do you find an octave that works for speaking
An "octave" a musical concept that is not linguistically relevant, even in tonal languages. You can use the same vocal range that you normally use when you speak your native language.
Most of the spoken resources available to me use female voices
There does indeed tend to be a gender bias in pedagogical materials, but of course there are many male Chinese teachers too. I can recommend 理查老师 (he live-streams a free lesson every day) and 大鹏.
when I try to imitate their pronunciation and tones I speak in a much higher voice than when I speak English and it causes a lot of strain.
It sounds like you might be a trained musician bringing some musical baggage into your language learning process. There's no need to match the exact pitch of the person you are imitating (or be an exact number of octaves lower or higher). The relative contour of tones is meaningful, but the exact pitch is not meaningful.
(But if you insist on trying to match an exact pitch, you could try using an audio editing program to modify the pitch of the speaker you are imitating up or down to match your comfort level.)
When I try speaking lower (what I feel is my normal English octave), I feel like I don't have a great range to distinguish different tones.
There are Chinese speakers that represent the full spectrum of vocal ranges. There is no natural human vocal range that is bad for distinguishing tones.