Hi Point sells any number of accessories for their carbines and pistols direct through their accessory website http://www.hi-pointfirearms.net/ . This includes magazines at a pretty cheap rate. Many firearms manufacturers charge $30-$40 a pop for factory mags for their weapons while Hi-Point sells theirs for much less, typically for around $15-$18. However, these mags are universally low capacity (less than 10 rounds.) This dates back to the 1994-2004 era Assault Weapons Ban that limited magazine capacities across the board to 10-rounds or less. Although the ban has sunsetted, some states like Massachusetts and California still have limits on magazine capacities and Hi-Point still sells mags like its 1994. From time to time different blogs have mentioned that the company is looking into developing hi-caps, but as of today, they still don't have them on their site. So until that day, let's look at the work arounds
Hi-Point's C9 pistol normally ships with a standard 8-round magazine that fits more or less flush with the bottom of the grip. The company offers a 10-round magazine that protrudes about 1.5-inches below the normal mag and adds a larger grip surface to the firearm.
ProMag aftermarket magazines
The main problem with high-cap mags for Hi-points is that they protrude from the grip...note the housing extension on the bottom of this promag.
ProMag of Southgate, California of all places, markets and sells several high capacity magazines officially for the Hi-Point carbines for $30-40 These carbine magazines are interchangeable with the Hi-Point pistols of the same calibers. No such compatibility was advertised by Hi-Point and the pistol mags do not fit the carbines in a vice-versa situation. ProMag makes a 15-round 9mm magazine that fits the 995 Carbine and 995-TS, as long as it does not have an ATI stock installed. They all use a follower, bumper/base plate and comfortable grip-sleeve made of an impact-resistant black polymer material; and uses heat-treated Chrome-silicon wire for reliable feeding and long spring life. The 15-round 40-caliber magazine extends 3.75" from the grip and only fits the newer 4095TS series. They also offer a 14-shot version for the .45ACP variant.
These magazines are hit and miss with some having no problems and others having to send them back as defective or modify feed lips. However, they seem to be the best bet for high-cap Hi-Point mags in most situations.
Modified 45ACP Carbine Magazines
One common work around for the Hi-Point 45 Carbine is to adapt hi-capacity Colt M1911-syle magazines to work in the firearm. These can be had in 15-20 round single stacks that extend below the grip of the carbine. They can often be had for about $20-$30. The two types of magazine's feed lips and body are similar, with the main problem being in the magazine release port. Some like in the video below, simply advocate bending out the release port on the magazine. This distorts the magazine but keeps your carbine intact.
One user even has a YouTube video of how to replace the magazine catch on the 4595 Carbine with a 1911-style catch of his own making (patenet pending!) This allows you to use standard 1911 mags of all sizes but modifies your firearm and may possibly void your warranty. Some have even gotten the 40-round Pro-mag 1911 drums to load into the bottom of the carbine. With some effort, you can make these extended magazines work in the .45 handguns as well.
Hopefully the company will come through with high-caps soon, but until then....