Any bulb in a 6500k spectrum with a length of 41.3 inches will work, you don't HAVE to buy pet bulbs.
But i have had a look where i bought my tubes from before (
www.lamspecs.co.uk) and those measurements in a daylight spectrum just arent available to buy in a cheaper tube im afraid, look at arcadia and juwel they make the best "pet" bulbs, and they do sizes that will fit your light bar.
There can be significant differences in intensity and even spectrum between different manufacturers regardless of the tube length, wattage or even spectrum. You wouldn't think a 6500K "X manufacturer" tube would be different from a 6500K "Y manufacturer" tube, but they can be as different as night and day.
I have used a great many T8 tubes as I have this lighting over all my tanks. When you are using 4-foot tubes, you have options that do not exist for shorter tubes. I use or have used 4-foot Phillips and Sylvania brands 6500K and 5000K T8 tubes (in combination, one 5000K and one 6500K) over my 4 and 5-foot tanks with good success; given that these tubes are significantly less expensive (about 1/5 or 1/6 the cost of the Life-Glo as an example) this is sensible. But, this does not apply to the shorter tubes, where Life-Glo is much more intense lighting; I've had the tank of plants die off under the less intense light, even though the specifications were full spectrum 6500K.
Life-Glo is a more intense light because of how it is made. The Power-Glo in the same series is more intense still, but here you run into the spectrum issue. I don't want to get into all that, so just leave it as there being quite a difference between tubes.
Another illustration...I tried the Aqueon "full spectrum" tube in 24 inch length, 20 watts. The intensity seems OK here, but the colour rendition is very "dull." The Life-Glo 24-inch 20w T8 produces a cleaner and crisper light than the Aqueon, though both are full spectrum. I've also used the Zoomed 6500K, and it is different still, closer to the Life-Glo though not quite as good.
I don't know anything about the Juwel tubes as I have never had a Juwel unit, and I have never seen Arcadia (may be a UK brand?). But bee careful, as you can buy this or that tube and not have anywhere near the same quality of light, whether intensity or spectrum, just going by data. It often takes actual use of the tubes over the aquarium, and over 8+ years I have tried all those I recommend or mentioned.