They mainly come in standard sizes, 54W are the standard size for 4' fittings and are dirt cheap (size 1149mm).
5' fittings run 35W, 49W or 80W lamps (1463mm) and they are cheapish as well.
14W or 24W fit in 2' fittings - they are 549 mm long and again pretty cheap as well, the reason they ae the same length is most "14W" ballasts are actually rated up to 24W.
A word of warning for the small lamps (like a lot of under-cabinet lamps) is that 13W & 14W are different lengths (525mm & 549mm)and the 13W actually runs of the same ballast as the 354mm long 8W T5's. This is easy to see if you look at a 13W and 14W fitting (with the same colour temperature tube), the 14W looks almost twice as bright.
As soon as you want non-standard sizes the price goes up (ask the owners of Juwel aquariums).
The postage is always going to be quite high on long fluorescent tubes as they are both fragile and awkward.
I usually try and have an occasional relatively large order to reduce the cost of p&p per item.
Could any of you kind people point me in the right direction for the best bulbs to get for a Juwel tank - tube size 30mm ( T5 size ? )
I was runing this set up in my old tank - T5 Fluorescent Tube 54W 840 Osram 54 Watt and T5 Fluorescent Tube 54W 880 Skywhite Osram 54 Watt - and would like to know what would give me similar effect etc?