I really want my vintage fixed gear to have that Retro look. The problem is British seat bags are priced out of my league. (Ouch!!) I saw in an old ad for Brooks that leather saddle bags was the in thing to have. Almost no one makes a leather saddlebag anymore so is there some way to convert a bag into what I want?

Ostrich makes really nice bags and shopping in Japan is good any time the JY is over 100 to the $
https://alexscycle.com/collections/saddle-rear-bags
Alex website seems to pop up a daily discount code as well.

especially looks good when you compare prices to US vendors for the same bags
Ostrich S-2 Canvas Saddlebag | Bike Touring News
Ostrich SP-731 Saddle Bag (Cream or Black)

These work best with some type of rear support like a Bagman, rear rack, or Nitto R-12 bag support.

I really like Zimbale's stuff (I've been using a front rack bag of theirs for years and also have a saddlebag that is pretty great but used less).
zimbale2001 | eBay
These are their completed auctions, but I've had good luck asking them to post a bag I like if there isn't a current auction for it.

How's World War II surplus for retro?

Make It: The $10 Bicycle Saddlebag | WIRED

The bag in the article is a British MkII gas mask bag. I bought one from eBay for about ten bucks.

Can second the surplus approach, swing by an army surplus store, find a bag that fits your tastes. They usually also have some straps that you can use. Old army knapsacks, ammo bags, and musette bags are pretty versatile and easily modified.

Actually the Classic British seatbag was/still is Carradice.
Made since the 1930's they are an extremely durable and practical weatherproofed canvas/leather construction.
There is a '70's production British seat bag on my town bike and a Barley model like the attached pic on my Audax-ish road bike.

Not inexpensive to acquire but on a $ per decade of use an excellent value.
From a practical and useful design aspect, better than any "modern" design for my use(s) and decidedly Retro style.
They come in a variety of sizes to suit any need.

I acquired my Barley at a reasonable price new from a British cycle goods company a few seasons ago.
The bey also has listings of used Carradice bags, it takes a lot of abuse to render one worn out.

https://www.carradice.co.uk/

edit: Note the drawing of a British club rider on his way to an event w/ "sprint" wheels on carriers and kit in a proper seatbag "back when".

-Bandera

If you search Rudge Pathfinder images, you'll get some nice catalog listings from the 50s that show a wide range of bags on clubman bikes, leather, Carradice-like and Ostrich-like.

Agreed.

These don"t wear out.

I have two Barleys, one of which I picked up at a local garage sale.

Although it was grimey, it works just fine.

In fact, I just put it on the back of my newly acquired CSI.

The newer one came from a shop in England we stopped in on the way home from a trip five years ago.

Dirt cheap, as it was on sale for a small amount.

As for the Ostrich bags, these are a great value.

I also use Alex's and purchased a rando style box for the front of my Bilenky.

It has been just fine so far.

Not on the level of a Swift or a Ruthworks, but it is value priced.

I was going to mention the Carradice Classics as well. Looks great on an older bike. I don't know about the never wear out comment though, on my second trip with mine I noticed the brake cable guide wore a hole straight though my saddle bag.

Never said "never".

One great thing about the Carradice design is their repair-ability w/ old school techniques and materials.
Recently I noticed that the dowel rod on my >40 year old seatbag was wearing through the end material (one too many frozen free range bug-fed chicken loads bouncing down the chip-seal).
Took it to a local saddle/tack shop (you could also take it to a shoe repair shop in a more urban setting ) had two leather reinforcements stitched in for $20: good for another several decades or so...

Repair-able not Disposable.

-Bandera

+ 1 on a used Carradice.