You have to be careful - antibiotics are NOT interchangable but
are targeted at specific bacterial types.  So it's not just
a matter of finding *an* antibiotic that's lactose free, it's the
right one (or right class of antibiotic).

I know that I have been prescribed Amoxicillan for sinus infections,
and I've had the druggists look through their various package inserts
for a non-lactose pill.  Interestingly, the generic was non-lactose and
the name-brand had lactose.

One of the best investments (providing you can lift about 20 pounds
fairly easily) is a PDR - Physician's Desk Reference.  It gives the
active and inactive ingredients for most prescription medicines (there is
also a PDR for OTC drugs, but it's not as extensive in my experience).
The one for prescription drugs does not always have details on generics,
however.  The PDR (printed form) costs about $60, and given the speed
at which pharmaceutical companies move, it's usually good for two or
three years before it's worth getting a new one.

You can also get the PDR in CDROM form, from:

https://www.pdr.net/login.jsp

I don't have it myself, but I might get it.  Works only on PC's
if that's an issue.  It includes both the prescription and non
prescription drugs' PDR.

Good Luck.