Mother of groom wedding outfit.

I've already got the shoes and jacket.

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.

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.

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

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.

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 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.