I went down this route myself about 2 months ago, finding various prices on cylinders and working out what I needed to put a fill station together for myself.
If you are using 4500 psi HPA then you're going to need a 300 Bar Dive Cylinder (around 4350 psi). The largest you can get in 300 Bar is a 12L Cylinder. You can get 15L in the cheaper 232 Bar (around 3300psi).
I ended up buying from UnderwaterWorld (
http://www.underwaterworld.co.uk/acatalog/Cylinders_Section.html) at Stoney Cove in Stoney Stanton. They don't list the 300 Bar cylinders on their online shop though. I had to ring for those but they had them in stock, where as everywhere else didn't and would have to order in for me. That was kind of lucky in the end as I'd have paid a higher price if other places had them in stock to get one quickly. As it happened I found the place with them in stock also happened to be the least expensive.
I was looking for one for our next trip to the local field so wanted somewhere I could collect. The highest I was quoted for a 12L 300 Bar was £274 at Midland Dive Equipment in Leicester. Dive Shop in northampton quoted £200 and UnderwaterWorld at £175 for 12L 300 Bar Faber Steel Cylinder.
I ended up buying 2 x 12L 300 Bar Cylinders with twinning bands to make a 2 stage cascade fill system for a total of £400. Since I collected them they filled both cyclinders for free on collection.
For a 300 Bar cylinder you'll need a 'DIN' Fill Station, rather than an 'A Clamp' one (to be used on 232 Bar Cylinders). I bought a DIN Fill Station (includes guage and purge valve) for £64 delivered from London International Paintball Supplies. I really need to get another as it is a pain to move it across each time you do a 2 stage fill.
The 12L 300 Bar with built-in Fill Station at LiPS is a good price at £240 when you consider even a well priced cylinder at £175 plus £64 for a DIN fill station £239. Also, you are guaranteed that this only needs testing every 5 years, rather than 2, as it CANNOT be used for diving as it doesn't have a standard dive valve. Even though you don't use the standard cylinder for diving I think you'll find it has to have a hydrostatic test every 2 years as it COULD be used for diving. You may find that if a standard cylinder isn't within a 2 year date then many Dive shops won't fill it.
With 2 12L Cylinders I've had around 14-15 fills so far before it's dropped below 3000 psi into my 1.1L Fibre Wrap HPA bottle. I reckon I can get about 20 fills before it gets down to just above 2000 psi.
Refill for a 300 Bar 12L Cylinder costs £3.50 at my nearest shop, Midland Divig Equipment, in Leicester. So, that's £7 for 15 paintball fills to 3000psi, going to around 20 useful fills until I get to about 2000 psi.
Hope that helps.
