If you're meaning uniform badges, then, as has been said, e-bay. There's a category collectables - transportation - railwayana - hardware railwayana - badges and emblems, currently with over 4,000 items for sale.
Buyer beware - I'm not all that expert in railway uniforms / badges, but some of the 'engineman cap badges' that came up on the first page look very much like reproductions that were made for sale to enthusiasts, rather than originals (i remember seeing them for sale at preserved railways etc in the past.)
Some e-bay sellers are (at best) not well informed about what they are selling. Very occasionally that means you can find a bargain because the seller doesn't really know how much something is worth and / or doesn't describe it in a way that would come up readily on a search for that precise item, or because they list it in the wrong category. More often it means unrealistic claims are made about an item.
There are a few railwayana auction firms out there who hold auctions a few times a year, but tend more towards the more vintage / high end of the market - and read the small print carefully as there's usually a buyer's premium (which in turn attracts VAT) added on top of the sale price, and you may have to arrange to go and collect something, and they may be fairly inflexible about times and will start charging storage fees if you don't collect within a short time frame after the auction - or you have to pay their preferred courier to pack and deliver it.
