Two versions of the game exist: Japanese and English. Both are sold via DLSite.come
Buying the English version
The direct link to the product is at http://eng.dlsite.com/work/=/product_id/RE023758.html
The cost of the software is 2,100 Yen (approximately USD $21.36 / EUR €16.39 / GBP £15.20).
Buying Japanese version
The direct link to the product is at http://maniax.dlsite.com/work/=/product_id/RJ023758 (requires age verification).
The cost of the software is 1,890 Yen, and may be purchased by an American credit card. Depending on the exact conversion rate of the purchase, the total should be slightly less than 20 US Dollars.
Updating to the latest version (Japanese only)
The purchased version from DLSite is at version 1.52. The maker's official website offers update patches to the latest version.
First you need to update the game from version 1.52 to 1.55. Then a second update is required to get it to version 1.69.