You can usually change yarn as long as you keep the same weight, so in this case just holding 4 strands of any bulky weight yarn should work. As far as using acrylic yarn, it's really just like any other kind, some people like it and others don't.
To make sure I got what they would like I'd ask them..... and ebay is a great place to buy yarn....
"A person is a person no matter how small" Horton Hears a Who
by Dr Seuss
Wife to GooB; Mom to Tori (7) and Belle (4) Jeremiah (2)
MamaDawn there too
My Little Shop