You'll spend more on shipping than you will save on price in almost every case. Find them locally if you can and transport them yourself. You can certainly shop price online and use the prices you find to negotiate your final price with local vendors. In fact, its a good idea to do this. You can tell the local merchant that you can get them online for $ X, and if he's willing to meet that price, you'll buy them from him. This way you get the online price, AND eliminate the shipping expense.
Of course, by now its no secret that I prefer the do-it-yourself round over buying an expensive kid. Check out the Convert-2-EV system. It has everything you need to find the right parts at the lowest cost. It also contains step by step instructions for completing your project while saving thousands of dollars over the cost of using a kit. These guys really know their stuff.
The guide is designed, written and illustrated specifically as a DIY project handbook. This way you'll also get instructions for finding low cost, reliable motors and batteries in your local area. UIltimately, the amount of work involved is the same and the project costs a fraction of what people usually spend on electric car kits.
OK, so we left off talking about how to research conversion kits and what questions to ask. Let's pick up from there.
If you know what model car you are using, and by this point you should, it is easy to estimate the weight using one of the many automobile rating sites, like Edmunds. Although the engine will be coming out you will be replacing that weight with an electric motor and batteries.
The motor that you replace it with, whether you buy an electric car kit, or do it yourself with a guide will weigh around 150 pounds. The batteries will weigh around 800 lbs, but again this depends on the type of battery you use.
So as a general rule you can figure that the electric motor and batteries combined will weigh approximately the same as the engine you are removing and go with the total vehicle weight for the car you are using. This should be close enough.
Next, make sure to find out exactly what kind of instructions are provided with the kit and how detailed they are. You need to ask specifically whether or not the instructions are written - and are intended for - someone who has never done a conversion before.
The bottom line here is will you be able to understand them and complete the project. Often these kit instructions are not really meant to be used by the average do it yourself enthusiast, so make sure they answer these questions. .
Next, make sure you find out what kind of batteries the kit equipment is meant to use. There may be some restriction and you won't be able to calculate the total cost of the project until you know this. Remember, when it comes to electric car kits, batteries are not included! Speaking of batteries, I think it is worth repeating that you should not buy your batteries online.