EMP strong enough to kill devices in an area >20 meters diameter
Let's do a bit of rough gathering of numbers:


Fry is a pretty relative term. Damaging a sensitive device that is actually designed to receive energy from electromagnetic waves is certainly much easier than to damage e.g. a fridge. So, let's stick with *intentional receivers. That mostly means phones, but maybe also things like drones, gate openers, intercom systems, and any other thing that communicates wirelessly.
Purchasable who can purchase what for which price? If you're the Pentagon, meh, you can buy pretty much anything that is technically possible on this earth. If you're John from the third floor who's never dealt with electronics design, you might not even figure out where to buy 1m of cable. Somewhere in between are people that actually design electronics. So, let's assume we're dealing with someone who's got access to the internet, sufficient knowledge, but needs to keep a low profile, so he won't be buying large quantities of explosives or the like.
Real weapon this bodes bad for your storytelling: Of course, whatever you build is a real weapon in the end. Much like an IED is a real weapon.


So, let's stay with phones: 

GSM phones are known to work well with a field strengths as high as \$80 \text{dB}\frac{\text{µV}}{\text{m}}\$, which is 10-2 V/m. So, to damage a phone reliably, let's assume you'd want to have 100 times that field strength. That's 1 V/m of E-Field strength!!

So, we take the field strength \$E\$ and relate it to the transmitted power \$P_T\$:

$$E = \frac{\sqrt{30P_T G_T}}{r}$$

With \$r=20\,\text m\$ being the radius of the sphere around the transmitter where this field strength is observed (i.e. the distance phone–EMP device) and \$G_T\$ the antenna gain, ie. a measure for how much the transmitter "focuses" the energy in one direction. Since you were talking about radii, let's assume it doesn't focus at all, but sends in all directions equally, so \$G_T\equiv1\$. Solving for the power leads to

$$\begin{align}
P_T &amp;= \frac{E^2r^2}{30G_T}\\
&amp;= \frac{1^2 \frac{\text{V}^2}{\text{m}^2}\cdot 20^2\,\text{m}^2}{30}\\
&amp;= \frac{400}{30}\text{W}\\
&amp;\approx 14\,\text{W}
\end{align}$$

Note that this is the power to distribute only over frequencies that the receiver actually picks up. So, when you'll build a 14 W transmitter that works exactly at the frequencies that a GSM phone uses, it will damage these – but not things that work at other frequencies! 

But accepting that restriction, we can state that yes, whilst not trivial, it's certainly not hard for someone with microwave design experience or good instructions and access to standard components (that being oscillators/frequency synthesizers, and power transistors) that can be freely purchased online, as well as copper-clad board and patience, to build something like that. 

If your question is: the protagonists lives in a house and needs to build such a device out of things he can find or buy at a supermarket: nahhh.

Your best bet would be to take a microwave oven, and remove the casing. Tadah, high-power, lethal, electronics-damaging microwave gun. Dangerous as hell. Wouldn't recommend doing. Of course, there's enough folks on youtube that don't effing care. These two dimwits have the disadvantage of living in a war zone, so they really have limited access to electronic components: