Mother of groom wedding outfit.

I didn't even by a special MoG outfit. I already had a nice outfit in my wardrobe, bought for a special event a year or so previously. I spoke to DS and soon to be DDiL and her mother and everybody was quite happy with that. The bride's mother and I also agreed that neither of us would wear hat.

I doubt anyone but the MoG and MoB even look at what they are wearing. Everyone else's eyes are on the bride, groom and retinue.

If you know the MOB, it would be advisable to have a word with her to make sure your don't clash with her outfit or even choose the same colour scheme. Take someone with you armed with a phone so that you can see a photo of yourself in the various dresses you try on. Good luck with that.

I’d also have a look at the MOB/ MOG section on EBay. Many of the outfits have been worn once for the occasion.
I sold my MOG complete outfit, worn once for a 10th of the original cost.

Try Phase 8 (I think they are in John Lewis). Roman online seems to specialise in the sort of dress you are looking for

Like you I hate the wedding type clothes people purchase. For one thing they are way too expensive and where do you wear them afterwards.
When my son was getting married in 2013 I couldn't think what to wear.
My daughter and future DIL came to the rescue and came out with me shopping for THE DRESS.
I bought a smart navy 50s type with short sleeve and below knee length.
I love it but would never have bought it if shopping on my own.
The good thing is I have had so much use out of it when we go cruising as it's smart but not what I call a wedding type dress.
Take someone with you is my advice.

Put what you want into google then press images or go to a store that has a personal shopper.Good luck, I can not look forward to an event until I have my outfit sorted.

I've already got the shoes and jacket.