Progrestrone supplements luteal phase defect

We have been TTC for close to a year now and had fertility testing done with Sims at the end of last year.

At the outset I'd expressed concern because I thought my luteal phase was quite short, about 10 or 11 days each cycle so maybe needed progesterone. But when I did the 21 day progrestrone test it came back at a high enough level they weren't concerned. However since then I have had two cycles were my luteal phase was only about 8 days. We are giving it a little bit more time before we formally go into the ivf process but I would really like to try progrestrone supplements to see if it helps as our other results were fine. I felt Sims jumped quite quickly to suggesting ivf so not sure they will prescribe it but are they my only route or have people gotten it from the gp for example?