Theo's moniker for all things underworld-related is Odin. Odin is well-known for being pretty much the guy you talk to if you need a disguise that's damn good.
There is only one person, aside from Theo, who knows who Odin is and that is Graham Pritchard. (It's hard to hide stuff like that from your best friend when he was your guinea pig during the years you were both in school together.)
Any inquiry about acquiring a better than just good disguise will bring up Odin's name near immediately from anyone who knows about glamours. Most of the underworld knows the name though few seem to have first-hand experience with him.
All owls addressed to Odin are re-routed to Theodore through a series of re-routings so they cannot be traced to him.
Odin sells disguises including and combining polyjuice and glamour trinkets. Polyjuice is the cheapest thing to purchase from him as it's the easiest to make. The glamours are a pretty damn penny if you want one. The most effective disguises combine both the trinkets and a polyjuice, though this is clearly very expensive.
Theo uses his day job to collect hairs from rather unremarkable people, wizards and muggles alike, to make his polyjuice from.
The polyjuice Theo makes will last longer than normal polyjiuice and require less to be ingested for that time period to be sustained.
A person's scent also changes with Theo's polyjuice, something that does not happen with normal polyjuice. This means even werewolves cannot tell who the person actually is.
Glamour trinkets are glamours attached to something which is always made of metal. In order for it to work the metal has to be touching skin. The glamour immediately comes off when skin contact is lost. If you're willing to pay more (and it's quite a bit more) he can make it so the glamour only works for a specific individual, though he essentially needs something with your DNA to do it (hair, skin or blood). For example, he made it so Penny's kitty cat glamour only works for her, he found a stray hair on the table where she'd been sitting with him in order to do that.
His glamours can be full-bodied, though he prefers not to because it's not a particularly effective disguise as it's still your own body under the illusion. Most often his glamours adjust voice, speech pattern, accent, facial expressions, body movement and add small habits or quirks to the persona, such as biting your nails, never looking up, playing with your hair. It is literally illusion so if the wearer never does things like play with their hair which are crafted into the illusion it will create it and if touched the game is up.
Requests can be made for specific types of people (i.e. Male, mid-thirties, black, shaved head, etc.).
No one knows what Odin looks like because he's always wearing one of his own disguises when he meets people (though he does not wear the trinkets as it's too risky, he casts the glamours directly onto himself). Often he will wear the same disguise when meeting a returning client as he did the first time (he keeps notes on what he wears and with whom).
Theo makes the best polyjuice and crafts the best glamours there are, period. No one does this better than he does and he's only known his glamours to be forcibly broken by another once, though people have tried much more often than that.
In order to make the best glamours he has to be able to test them, this means he's a bit like a particularly skilled craftsman. Average glamours are laughably obvious to him, he can see all the flaws in them, though he obviously can't see through them. He can break them, and most of them he finds are rather easy to break. He hasn't ever found a glamour he, personally, couldn't break. He is the only person he knows who can consistently break the glamours he crafts.
Polyjuice requests are available immediately if he has the physical type desired. Glamours have a waiting period as they are often custom crafted for the particular client's request. Some more generic glamours are available upon request and are slightly cheaper as Theo simply does them in his free time and not under a time limit.
You still need to act the part. His disguises will do a lot of the work for you, but if you can't act the part you're still fucked.