Well, if money is no object ... you
could order a hand-built one from Peter Dyson & Son in England - though I suspect you won't want to pay quite as much as their £2500 starting price (C$5000) - especially since you could likely find a shootable
original for that much! ...
I think that IMA only has Brunswick-pattern percussion rifles, and I don't see a Baker rifle in the Pedersoli line. I'm not sure anyone is actually producing a
rifled reproduction Baker for the market (except perhaps custom gunmakers.)
Although you may already be aware of them, there are at least two
Canadian sources for the relatively low-cost smoothbore version of the Baker - for which "rifling options" do exist -
Discriminating General - Baker
This .62 cal. (as opposed to .69 - .70, which I understand was the other "standard" bore size of originals) smoothbore repro is supposedly made by the company which made the repros for the Sharpe's series - and they claim that Bernard Cromwell, the author of the Sharpe novels, bought his rifle from them. Nice looking, judging from the pictures. You will note that if you buy from them, they will put you in touch with a US-based gunmaker who will make, fit and install a rifled barrel (for US$150, plus shipping.) I take it from this that you'd still have the smoothbore barrel as well (which comes with the vent hole not drilled - though that is easily enough remedied, of course.) Their current price is C$679, and they also offer a repro sword bayonet and scabbard (apparently only available if you buy a "rifle") at C$140 .... I have dealt with these people for some stuff in the past (though not firearms) and have no complaints.
The other source:
Loyalist Arms & Repairs
This is also a smoothbore version - at C$723 - and is also bored in the smaller .62 caliber.
As an alternative to seeking out a rifled barrel for one of these smoothbores, I seem to recall reading that the thicker-walled .62 cal barrels lend themselves quite well be being rifled, if you have access to a gunsmith who can do that for you ....