Google, Amazon, and random people with 3d printers are your friends for this.
Name brand wise you're going to want to look For Marx and Tamiya toys / kitz.
Marx is the far more economical option, yielding stuff like this:
http://www.ebay.com/itm/Vintage-1960s-Marx-Silver-Plastic-Long-barreled-Howitzer-Military-Toy-Weapon-/131779526992?hash=item1eaeac0150:g:MoAAAOSwu1VW4HA0
http://www.ebay.com/itm/Vintage-Toy-Marx-Lumar-Us-Army-Howitzer-Field-Canon-1950-s-/172155924820?hash=item28154af154:g:HvwAAOSwoBtW5bIB.
Functional and useable, but visibly a toy. Granted with a little bit of time and a few bits you can easily prep (and then pain) them in such a way that it won't be clear they're childrens toys, and given the money you save their generally worth that investment.
If you're looking for something a bit.. Higher profile and don't mind paying a bit more, consider Tamiya. To give you an example of their quality:
http://www.ebay.com/itm/Tamiya-37022-German-Bundeswehr-Self-Propelled-Howitzer-M109A3G-1-35-scale-kit-/391421851238?hash=item5b228f7666:g:gecAAOSwbwlW~gsl
These are actual model kits, so they will cost more money. However they're still leaps and bounds cheaper than Forgeworld and quite nice. So again with minimal work (worth it due to money saved) they can be
40k-iffied with ease.
For some specific examples of stuff.
Use Nebelwerfer kits for your quad guns.
http://www.ebay.com/itm/Italeri-German-Nebelwerfer-Rocket-Launcher-Model-Kit-324-/182090677734?hash=item2a65733de6:g:avwAAOSwq19XDQfp
They're what inspired the quad gun design, look nifty, and are far cheaper.
Just typing what you want into Amazon / ebay will yield you tons of results. I got both of the following by typing "howitzer toy" into Amazon, and they're pretty passable heavy mortar / even Earthshaker / Medusa sub-ins depending on how you feel.
http://www.amazon.com/Military-Combat-Accessories-Single-Howitzer/dp/B01704QZXE/ref=sr_1_fkmr0_2?ie=UTF8&qid=1460879675&sr=8-2-fkmr0&keywords=marx+howitzer+toy
http://www.amazon.com/Toxso-Model-M102-105mm-Howitzer/dp/B011DXZSYU/ref=sr_1_fkmr0_1?ie=UTF8&qid=1460879675&sr=8-1-fkmr0&keywords=marx+howitzer+toy
Lastly, if you're looking to actually spurge but not go crazy with
FW prices, Maximini and Kromlech each offer some fan-fething-tastic Medusa Siege Gun alternatives. Which is really stellar, because from experience I can assure you that the Medusa is far and away the hardest thing to find an alternative for.
Maximini:
http://cdn.bols.netdna-cdn.com/wp-content/uploads/2011/01/siegegun_cast.jpg
http://maxmini.eu/siege-gun-heavy
http://maxmini.eu/siege-gun-gunonly-heavy
Kromlech:
http://bitsofwar.com/home/144-imperial-siege-mortar.html