You don't need to get bulbs from an aquarium supplier. I don't believe aquarium companies actually manufacture bulbs, they might just select from the cheapest possible options. Find a reputable medical germicidal bulb supplier and you will likely find bulb that fits your exact unit with high UV output and possibly even cheaper. i have an old Current USA UV unit and I've never bought a bulb from them. I just buy a standard UV germicidal bulb from a medical supplier and there are no issues with fit or anything else.
Cheers,
FB