You can't get a demand valve system without a prescription. Sometimes there are some for sale on eBay, and maybe other places.
You can get 25 lpm regulators at lots of places--amazon and eBay among them. You have to be careful what you buy, though, because different size tanks take different types of regulators. The smaller tanks use a CGA 870 regulator, link the one pictured here: http://www.amazon.com/Medline-HCS8725M-Oxygen-Regulator-Latex/dp/B00BLQKKQQ?ie=UTF8&keywords=cga%20870%20regulator&qid=1465345545&ref_=sr_1_4&sr=8-4
Larger tanks use a CGA 540 regulator that will often look like the one pictured here: http://www.amazon.com/Medline-HCS5408M-Best-Valued-Regulators-Connector/dp/B00BLQKI86?ie=UTF8&keywords=cga%20540%20regulator&qid=1465345653&ref_=sr_1_1&sr=8-1
Of course, you can look at the regulator on your current tank to see which you would want right now. However, if what you have now is a tank that takes an 870 -- that is, a smaller tank -- it's likely that you are ultimately going to want a larger tank, unless you have several small ones. Depends, of course, on how many hits hubby gets each day and how long it takes to abort them. You use more O2 at 25 lpm, but the abort is faster. AND, if you're going to get a "25 lpm" 540 regulator -- the kind for the larger tank -- you might as well get one with unlimited lpm, which would be a welding regulator. The most commonly recommended one of those is this one: http://www.harborfreight.com/catalogsearch/result?q=oxygen+regulator However, I notice that amazon has one that looks the same for less $: this one: http://www.amazon.com/IMAGE%C2%AE-Welding-Welder-Regulator-Cutting/dp/B00JP9WIF2?ie=UTF8&keywords=cga%20540%20regulator&qid=1465345653&ref_=sr_1_2&sr=8-2
Welding regs don't have clickable lpm gauges like medical regs, so you have to fuss with it a little to get it set right -- but it does provide very high lpms if they are needed.
The only reason to use a higher-lpm regulator is if he has to wait for the bag on the mask to fill before breathing in. It allows for a more forceful breathing pattern while still having the O2 ready for the next breath.