There are many places where one can buy new or used stationary bikes. For the beginning you would need to decide do you need new stationary bike or used stationary bike. Then you need to figure out your budget and what type of stationary bike you would like to buy. After those steps, everything is a matter of searching the internet online shops or real shops and when finding the appropriate model to order it.

Here are some collection of sites and real stores where you can order yourself a used stationary bike:

http://sporting-goods.shop.ebay.com/Exercise-Bikes-/58102/i.html?rt=nc&_catref=1&_dmpt=Exercise_Fitness_Cardio&_trksid=p3286.c0.m282
Here you need to pick condition used.

http://www.amazon.com/Exercise-Bikes-Aerobic-Training-Machines/b/ref=amb_link_84596571_5?ie=UTF8&node=3407781&pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-7-0&pf_rd_r=03ZR83J190B5JFBSMCFX&pf_rd_t=101&pf_rd_p=1090151022&pf_rd_i=3407731

Just bellow price there is a note saying is there any used offered. If yes, click it.

You can also check http://www.craigslist.org/ to see is there any seller nearby for quick purchase.

For ordering a new stationary bike besides checking above links check also this page http://www.usedstationarybikes.org/stationary-bike-brands.html

You have there list of stationary bike brands and links which point to their official sites. On every site you can order online some of their models and also check what is the nearest store where you can directly buy stationary bike.